Wastewater Treatment Technology

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This section covers the characterization of wastewater, effluent discharge standards, sewerage systems and the types of sewer, the estimation of flow in sewers, wastewater treatment design flow rates, the physical, chemical and biological operations available, the classification of treatment stages and the layout of a treatment plant.

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Characterization of Wastewater • Wastewater is about 99.9 per cent water by mass; the whole of treatment is concerned with the remaining 0.1 per cent. • Physical characteristics — total solids, divided into suspended and dissolved, and each of these into volatile (organic, lost on ignition at 550 °C) and fixed (mineral); temperature, colour, odour and turbidity.

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Settleable solids are measured in an Imhoff cone over one hour. • Chemical characteristics — the organic load, measured as BOD, COD or TOC; nitrogen as ammonia, organic, nitrite and nitrate; phosphorus; chlorides; sulphate; alkalinity; pH; oil and grease; and heavy metals and specific toxic organics in industrial wastes. • Biological characteristics — bacteria, viruses, protozoan cysts and helminth ova, assessed in practice through the indicator organisms, total and faecal coliform and E. coli. • The oxygen demand measures must be clearly distinguished, as this is a standing examination item: • BOD5 — the oxygen consumed by micro-organisms in oxidising the biodegradable organic matter, measured over five days at 20 °C.

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It measures only what is biodegradable, takes five days, and is sensitive to toxicity in the sample. • COD — the oxygen equivalent of the matter oxidisable by dichromate in acid, obtained in three hours or less.

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COD is always greater than BOD, because it oxidises the non-biodegradable fraction as well. • The ratio BOD5/COD is the index of biodegradability: above about 0.5 the waste is readily amenable to biological treatment, and below about 0.3 it is not, which is the usual reason for choosing a physico-chemical process for an industrial effluent. • Ultimate BOD and the first-order relation:

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BODt = L0(1 − 10−kt), and for domestic sewage BOD5 is about 0.68 of the ultimate BOD at a rate constant k of 0.1 per day to base ten.

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Constituent Weak Medium Strong (mg/L) Total solids 350 720 1200 Total suspended solids 100 220 350 BOD5 at 20 °C 110 220 400 COD 250 500 1000 Total nitrogen as N 20 40 85 Total phosphorus as P 4 8 15 Oil and grease 50 100 150 Total coliform, per 100 mL 10⁶ to 10⁷ 10⁷ to 10⁸ 10⁷ to 10⁹ Effluent Discharge Standards • Standards are of two kinds and must not be confused: a stream standard fixes the quality of the receiving water, while an effluent standard fixes what may be discharged.

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An effluent standard is simple to enforce but takes no account of the assimilative capacity of the stream; a stream standard is rational but difficult to administer among many dischargers. • Nepal applies generic standards for industrial effluent discharged into inland surface water, commonly cited as BOD5 50 mg/L, total suspended solids 50 mg/L, pH 5.5 to 9.0, oil and grease 10 mg/L and temperature not more than 40 °C, with additional specific standards for particular industries and a separate standard for discharge into a public sewer.

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Standards for the effluent of municipal wastewater treatment plants are notified separately.

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All of these are revised periodically and the current gazette must be consulted. • Discharge to a public sewer is permitted at weaker quality than discharge to a watercourse, because the sewer leads to further treatment; what matters there is that the discharge must not damage the sewer, endanger workers or inhibit the biological process downstream.

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Sewerage Systems and Estimation of Flow System What it carries Assessment Combined Sanitary sewage and storm water in one pipe One set of pipes, so cheaper to build and self-flushing in storms; but very large sewers, dilute flow that is costly to treat, and combined sewer overflows that discharge raw sewage in heavy rain Separate Two pipes: sanitary and storm independently Small sanitary sewers, strong and uniform flow to the plant, no overflow of sewage; but higher capital cost, two sets of pipes, and risk of wrong connections Partially separate Sanitary sewer also takes roof and yard drainage A compromise; the sanitary sewer is self-flushing and roof drains are simple, but the plant must cope with some storm flow • Dry weather flow is estimated from the water supply: about 70 to 80 per cent of the water supplied returns as sewage, the balance being consumed, lost to irrigation and lost by evaporation. • Infiltration of groundwater through joints and defects, and inflow from illicit roof and yard connections, must be added, and in an old system this can be a large fraction of the total. • Peak flows are obtained with a peaking factor that decreases as the population served increases, because the individual peaks of many users coincide less and less.

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The Harmon formula is the usual expression: peaking factor = 1 + 14/(4 + √P), with P the population in thousands. • Worked illustration.

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For a population of 100 000, P = 100, so factor = 1 + 14/(4 + 10) = 1 + 1.0 = 2.0.

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For a population of 10 000 the factor is 1 + 14/(4 + 3.16) = 2.96.

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The smaller town therefore has the higher peaking factor. • Minimum flow is also needed, since the sewer must still be self-cleansing at the lowest flow; it is commonly taken as one-third to one-half of the average. • Storm flow is estimated by the rational method, Q = C i A / 360 with Q in m³/s, i in mm/h and A in hectares, using a rainfall intensity for a return period of two to five years for ordinary areas. • Design flow rates for the treatment plant itself are a set, and each unit is sized on the appropriate member of it: the average day flow governs the biological process and the sludge production; the maximum day flow governs the sludge handling; the peak hour flow governs the hydraulics — channels, pipes, weirs and pumps — and the minimum flow governs the turndown of the measuring devices and of the chlorination.

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Unit Operations, Treatment Stages and Layout • Physical unit operations — screening, comminution, grit removal, flow equalization, sedimentation, flotation, filtration and gas transfer.

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These depend on physical forces. • Chemical unit processes — coagulation and flocculation, chemical precipitation of phosphorus and metals, neutralisation, adsorption, oxidation and disinfection.

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These add a reagent and generate a chemical sludge. • Biological unit processes — aerobic, anoxic and anaerobic conversion of organic matter and nutrients by micro-organisms.

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They are the cheapest means of removing dissolved organic matter and are therefore the heart of a municipal plant.

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Stage Purpose Typical units Preliminary Protect the plant from damage and abrasion Screens, grit chamber, comminutor, flow measurement, equalization Stage Purpose Typical units Primary Remove settleable solids and floating matter Primary sedimentation tank, with or without chemicals Secondary Remove dissolved and colloidal organic matter Activated sludge, trickling filter, MBBR, ponds, with secondary clarifier Tertiary or advanced Remove nutrients, residual solids and pathogens Nitrification and denitrification, phosphorus removal, filtration, disinfection Sludge handling Reduce the volume and stabilise the solids Thickening, digestion, dewatering, disposal or reuse • Layout principles: the plant is laid out so that the whole flow passes by gravity, which requires a hydraulic profile drawn from the outfall backwards, since the available head is fixed by the river level.

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Units are arranged in the order of treatment with the shortest possible interconnecting channels; duplicate units and bypasses are provided so that any unit can be taken out for maintenance; and the site is placed downstream of the town, below the lowest point of the sewer system, above the flood level, and where the prevailing wind carries odour away from habitation.

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Room for the inevitable future extension should be left from the beginning.

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This section covers the shapes of sewer and the reasons for each, the design criteria for a sanitary and a storm sewer, sewer hydraulics and Manning's equation, the hydraulic elements and partial flow diagrams for circular sewers, the construction of sewers and the materials used.

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Shapes of Sewer Shape Where used Reason Circular Almost all sanitary sewers Greatest hydraulic radius for a given area, easily manufactured, strong under external load Egg-shaped (ovoid) Older combined sewers Narrow invert keeps velocity up at low flow, so it remains self-cleansing in dry weather Horseshoe Large trunk and outfall sewers Flat invert and wide section; suits tunnelling and large discharges Rectangular and box Large storm drains, cut and cover Simple to build in reinforced concrete; easy to inspect Semi-elliptical Very large sewers in good ground Wide base spreads the load over soft foundations U-shaped and parabolic Storm drains Combine a narrow invert for dry weather with a wide top for storm flow Design Criteria • Self-cleansing velocity.

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A sanitary sewer must achieve at least 0.6 m/s, and preferably 0.8 m/s, at least once a day, so that the organic and grit solids are carried along rather than deposited.

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Deposition leads to blockage and to septicity, which generates hydrogen sulphide and causes crown corrosion of concrete sewers. • Limiting velocity.

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The velocity should not exceed about 3 m/s, because grit in suspension abrades the invert; in very steep ground drop manholes are used to dissipate the head rather than allowing the velocity to rise. • Depth of flow.

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Sanitary sewers are designed to flow at about half to three-quarters full at the design peak flow, which leaves capacity for underestimation and provides ventilation to carry away sewer gases.

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Storm sewers are designed to flow just full, because the design storm is itself an accepted risk. • Minimum size.

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A public sewer is not made smaller than 150 mm, and 200 mm is the usual minimum in practice, because anything smaller blocks on the rags and solids that inevitably enter. • Minimum cover of about 0.9 to 1.2 m under roads protects the pipe from traffic load, and the sewer must lie below the water main and, where they cross, beneath it, so that a leak cannot contaminate the supply. • Manholes are provided at every change of direction, gradient, size or level, and on straight runs at 30 m for small sewers up to 90 to 150 m for large ones, for inspection and cleaning.

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A drop manhole is used where an incoming sewer is much higher than the outgoing one. • Appurtenances: inverted siphons to pass beneath an obstruction, flushing tanks at dead ends, catch basins on storm inlets, and ventilating columns.

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Sewer Hydraulics • Sewers are designed as open channels flowing partly full under gravity, even though the pipe is closed.

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Manning's equation is used: • V = (1/n) R2/3 S1/2, and Q = A V, where R = A/P is the hydraulic radius and S the slope. • For a circular pipe flowing full, R = D/4, so Vfull = (1/n)(D/4)2/3 S1/2. • Typical Manning's n:

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0.013 for concrete and vitrified clay, 0.011 for PVC and HDPE, 0.015 for brick, and 0.013 for ductile iron.

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Note that n is taken as constant with depth in ordinary design, although it in fact varies slightly. • Worked illustration.

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A 300 mm concrete sewer at a slope of 1 in 400, n = 0.013.

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R = 0.3/4 = 0.075 m;

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V = (1/0.013)(0.178)(0.05) = 0.68 m/s, which satisfies the self-cleansing requirement.

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A = π(0.15)² = 0.0707 m², so Qfull = 0.0707 × 0.68 = 0.048 m³/s, that is 48 L/s. • The minimum slope is therefore fixed by the self-cleansing requirement, and a rule of thumb for small sewers is a gradient of 1 in D, with D the diameter in millimetres — 1 in 150 for a 150 mm sewer, 1 in 300 for a 300 mm sewer.

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Larger sewers need flatter slopes for the same velocity because the hydraulic radius is larger.

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Hydraulic Elements and the Partial Flow Diagram • The partial flow diagram plots the ratios v/V, q/Q, a/A and r/R against the depth ratio d/D, where lower case denotes partial flow and upper case full flow.

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Its four key features must be memorised, because they are asked in almost every examination: • 1.

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The velocity is a maximum at about d/D = 0.81, where v/V is about 1.14.

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A sewer flowing at four-fifths depth runs faster than the same sewer flowing full. • 2.

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The discharge is a maximum at about d/D = 0.94, where q/Q is about 1.08.

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A sewer flowing just short of full carries about eight per cent more than when it is full. • 3.

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At d/D = 0.5 the velocity equals the full-bore velocity, v/V = 1, while the discharge is exactly half, q/Q = 0.5.

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This happens because at half depth both the area and the wetted perimeter are halved, so the hydraulic radius is unchanged. • 4.

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At d/D = 1.0 the ratios return to v/V = 1 and q/Q = 1. • The physical reason for the maxima is that as the depth rises above about 0.8 D the wetted perimeter grows faster than the flow area, so the hydraulic radius — and with it the velocity — begins to fall again. • Practical consequence: because the discharge curve is flat near the top, a sewer should never be designed to run exactly full; a slight surcharge gives no extra capacity and removes the ventilation space. • Worked illustration.

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The 300 mm sewer above has V = 0.68 m/s and Q = 48 L/s full.

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At d/D = 0.5, v = 0.68 m/s and q = 24 L/s.

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At d/D = 0.81, v = 1.14 × 0.68 = 0.78 m/s.

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At d/D = 0.94, q = 1.08 × 48 = 52 L/s. • Where self-cleansing at minimum flow is to be checked, the Camp-Shields approach is used, which relates the required tractive force on the particle to its size and specific gravity, and it shows that the velocity needed to move grit of specific gravity 2.65 is greater than that needed for organic solids of specific gravity about 1.2.

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Construction and Materials Material Typical use Remarks Reinforced concrete (RCC) Medium and large sewers Strong and economical in large sizes; liable to crown corrosion by sulphide unless lined Vitrified clay Small house and lateral sewers Excellent chemical and abrasion resistance; brittle, heavy, many joints PVC and HDPE Small and medium sewers Light, corrosion-free, smooth (low n), long lengths and few joints; needs careful bedding and is damaged by heat Ductile iron Pressure mains, siphons, exposed crossings Very strong; needs internal lining against corrosion Brick masonry Old large sewers Built in situ to any shape; rough, slow to build, now obsolete for new work Steel Rising mains and crossings High strength and ductility; must be protected both sides GRP Large sewers and linings Corrosion-free and light; relatively costly • Construction sequence: setting out and transfer of levels with sight rails and boning rods; excavation of trench with timbering or shoring; dewatering; preparation of the bed and provision of a granular or concrete bedding; laying from the downstream end upwards with the socket facing upstream; jointing; testing; and backfilling in compacted layers. • Laying begins at the lower end so that any water in the trench drains away from the work and the pipe already laid is not disturbed. • Testing is by water test, typically 1.5 m head for 30 minutes with a permitted make-up, or by air test; and the line and level are checked by lamp and mirror or by closed-circuit television. • Crown corrosion is the characteristic failure of concrete sewers: septic conditions in the flow release hydrogen sulphide, which is oxidised by Thiobacillus bacteria on the moist crown above the water line to sulphuric acid, which destroys the cement.

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It is prevented by maintaining self-cleansing velocity and ventilation, and cured by lining with PVC, epoxy or vitrified clay.

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This section covers screening and the calculation of screen head loss, grit removal by horizontal flow, aerated and vortex chambers, flow equalization tanks, primary sedimentation and the design of each of these units, with worked examples.

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Screening Type Clear opening Purpose Coarse (trash) rack 50 to 150 mm Timber, drums and large debris; protects pumps Medium bar screen 20 to 50 mm The usual plant screen; removes rags, plastics, sanitary items Fine screen 6 to 20 mm Protects membranes and fine equipment; removes much more material Micro-screen Below 0.5 mm Tertiary polishing; removes algae and fine solids • Bar screens are set at 45 to 60 degrees to the horizontal for manual cleaning and 60 to 90 degrees for mechanical cleaning, the rake being driven by a timer or by the head loss across the screen. • The approach velocity in the channel ahead of the screen must be at least 0.4 m/s at minimum flow, so that grit does not settle, and not more than about 0.9 m/s at peak, so that the retained screenings are not pushed through.

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Velocity through the bars is kept below about 0.9 m/s. • Head loss through a clean bar screen is given by the Kirschmer equation: hL = β (W/b)4/3 hv sin θ, where β is a bar shape factor (2.42 for a sharp-edged rectangular bar), W the bar width, b the clear spacing, hv the approach velocity head and θ the angle to the horizontal. • A simpler and more often used form expresses the loss through the clogged screen: hL = (1/0.7)(v² − V²)/2g, where v is the velocity through the openings and V the approach velocity, with 0.7 an empirical discharge coefficient. • Worked illustration.

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Velocity through the bars 0.9 m/s, approach velocity 0.6 m/s: hL = (1/0.7)(0.81 − 0.36)/19.62 = (1/0.7)(0.0229) = 0.033 m, that is 33 mm.

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Screens are normally cleaned before the loss exceeds about 150 mm. • Screenings are produced at about 0.005 to 0.05 m³ per 1000 m³ of sewage depending on the bar spacing, and are disposed of by burial, incineration or landfill after draining.

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A bypass channel with a manually raked screen must always be provided.

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Grit Removal • Grit is the inorganic fraction — sand, gravel, cinders, egg shells, coffee grounds, bone chips — of specific gravity about 2.65 and size above about 0.2 mm.

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It is removed because it abrades pumps and pipework, accumulates in tanks and digesters and consumes digester volume without contributing gas. • The design problem is selective: the grit must settle while the organic solids, of specific gravity only about 1.2, remain in suspension and pass forward to the biological process.

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Removing the organics with the grit would waste treatable material and produce a putrescible deposit. • The horizontal flow grit chamber solves this by holding the horizontal velocity constant at about 0.3 m/s, which is the scour velocity for organic particles but not for grit.

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The detention time is 45 to 90 seconds, that is just long enough for a 0.2 mm grit particle to settle through the depth. • The velocity is kept constant despite varying flow by a proportional flow weir — a Sutro weir — or a parshall flume at the outlet, whose shape makes the depth of flow proportional to the discharge so that the cross-sectional area varies with Q and the velocity does not.

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This is the key design idea of the unit and is regularly examined. • Worked illustration.

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A grit particle of 0.2 mm and specific gravity 2.65 settles at about 0.023 m/s.

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For a chamber 1.0 m deep the settling time is 1.0/0.023 = 43 s, so at a horizontal velocity of 0.3 m/s the length required is 0.3 × 43 = 13 m, to which an allowance of 25 to 50 per cent is added for inlet and outlet turbulence, giving about 17 m. • The aerated grit chamber imposes a spiral roll with diffused air along one wall.

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The spiral velocity, not the forward velocity, controls the separation, so the unit is insensitive to flow variation and needs no proportional weir; detention is 2 to 5 minutes; and the air scours organic matter off the grit, giving a cleaner deposit.

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It also pre-aerates the sewage and releases odour, which is its chief disadvantage. • The vortex grit chamber uses a forced vortex in a cylindrical tank, is compact, has a small footprint and low head loss, and is now the common choice for new plants. • Grit quantities are about 0.004 to 0.20 m³ per 1000 m³ of sewage, very much higher in combined systems and on unpaved roads, which is the Nepali condition.

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Flow Equalization • An equalization tank damps the variation in flow and in load over the day, so that the units downstream see a steadier condition.

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The benefits are a smaller and more stable biological process, improved settling in the clarifiers, uniform chemical dosing, and the dilution of shock and toxic loads.

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The costs are the tank itself, the land, the mixing energy needed to keep solids in suspension and aeration to prevent septicity, and usually a pumping lift. • It may be placed in-line, with the whole flow passing through, which gives the best damping of load; or side-line, taking only the excess above a set rate, which is cheaper but damps the flow only and not the concentration. • The volume is determined from the cumulative inflow mass diagram: plot cumulative inflow against time over the design day, draw the straight line of cumulative average flow from origin to the end of the day, and the required volume is the sum of the greatest vertical distance above the line and the greatest below it. • Worked illustration.

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If the greatest positive deviation of the cumulative inflow from the average line is 420 m³ and the greatest negative deviation is 310 m³, the theoretical volume is 420 + 310 = 730 m³, to which 10 to 20 per cent is added for the unusable volume below the mixers and for freeboard, giving about 850 m³.

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Primary Sedimentation • The primary tank removes settleable solids by quiescent settling, and it is the surface overflow rate — not the detention time — that governs the removal of discrete particles. • Surface overflow rate SOR = Q / A, which has the units of a velocity and is the settling velocity of the slowest particle that is completely removed.

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Every particle with a settling velocity greater than the SOR is removed regardless of the depth of the tank, which is the ideal settling theory of Hazen. • It follows that for discrete settling the removal depends on the tank area and not on its depth, and that is why shallow tray settlers and tube settlers work.

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In practice depth matters, because it governs the detention time available for flocculation and guards against scour of the settled sludge.

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Parameter Typical value Note Surface overflow rate, average flow 30 to 50 m³/m²·d The controlling parameter Surface overflow rate, peak flow 80 to 120 m³/m²·d Checked separately Detention time 1.5 to 2.5 h Usually 2 h Weir loading rate 125 to 250 m³/m·d Limits the approach velocity at the weir Side water depth 3.0 to 4.5 m Circular tanks at the deeper end Removal of suspended solids 50 to 70 per cent 60 per cent typical Parameter Typical value Note Removal of BOD5 25 to 40 per cent 30 per cent typical; the rest is dissolved With coagulant added 80 to 90 per cent SS, 50 to 80 per cent BOD Chemically enhanced primary treatment • The central examinable point is why BOD removal is so much lower than solids removal: only the particulate BOD can settle, and roughly half of the BOD in domestic sewage is dissolved or colloidal.

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No amount of detention time will settle dissolved BOD, which is the business of the biological stage. • Worked illustration.

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A town produces 8000 m³/d of sewage with 220 mg/L of suspended solids.

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At an SOR of 40 m³/m²·d the area required is 8000/40 = 200 m², that is two circular tanks of about 11.3 m diameter.

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At a depth of 3 m the volume is 600 m³ and the detention time is 600 × 24/8000 = 1.8 h, which is acceptable.

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At 60 per cent removal the solids captured are 8000 × 220 × 0.6 /1000 = 1056 kg/d. • Four zones are recognised in a settling tank — inlet, settling, sludge and outlet — and the function of the inlet baffle and the outlet weir is to spread the flow uniformly so that the whole of the settling zone is used.

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Short-circuiting, in which part of the flow passes straight from inlet to outlet, is the commonest cause of poor performance and is detected by a tracer study. • Scour of the settled sludge is checked by the Camp formula, and the horizontal velocity is kept low enough that the deposited sludge is not re-entrained. • Primary sludge is 4 to 8 per cent solids from a circular tank, is highly putrescible and must be removed continuously or at least daily.

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This section covers the microbiology of the treatment process, the kinetics of microbial growth and substrate utilisation, yield and endogenous decay, solids retention time and the food to microorganism ratio, aerobic, anaerobic and anoxic processes, the types of reactor, and the distinction between suspended and attached growth.

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Process Microbiology • Bacteria are the principal agents of treatment.

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Heterotrophic bacteria use organic carbon as both energy and carbon source and are responsible for BOD removal; autotrophic bacteria use inorganic carbon and obtain energy from an inorganic compound, and the nitrifiers are the important example. • Protozoa, principally the ciliates, graze on the dispersed free-swimming bacteria and so polish the effluent; their presence in the mixed liquor is a reliable sign of a healthy, well-aerated sludge of adequate age. • Rotifers appear only at long sludge ages and indicate a very stable, well-stabilised sludge. • Filamentous organisms are a normal and necessary component, forming the backbone of the floc; in excess, caused by low dissolved oxygen, low food to microorganism ratio, nutrient deficiency or septic feed, they bridge between flocs and cause sludge bulking. • Fungi tolerate low pH and low nitrogen and dominate when the waste is acidic or nitrogen-deficient; they settle badly, so their dominance is undesirable. • Algae matter only in ponds, where they supply oxygen by photosynthesis. • The nutrient requirement for aerobic biological treatment is commonly expressed as BOD :

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Domestic sewage comfortably exceeds this, but many industrial effluents — sugar, distillery, paper — are deficient and must be supplemented.

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Kinetics of Microbial Processes • Growth follows the Monod relation: μ = μmax S / (Ks + S), where μ is the specific growth rate, S the limiting substrate concentration and Ks the half-saturation constant, the substrate concentration at which μ is half of μmax. • The shape of the relation is the key to the whole subject: at high substrate concentration, S much greater than Ks, growth is zero-order and proceeds at μmax independently of S; at low concentration, S much less than Ks, growth is first-order in S.

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This is why a plug-flow reactor, in which the concentration falls along its length, behaves differently from a completely mixed one held at the low outlet concentration. • Net growth allows for endogenous decay: μnet = Y U − kd, where Y is the yield coefficient, the mass of cells produced per unit mass of substrate used (typically 0.4 to 0.6 kg VSS per kg BOD for aerobic heterotrophs and only 0.05 to 0.1 for anaerobes), U the specific substrate utilisation rate, and kd the endogenous decay coefficient, about 0.04 to 0.075 per day. • The low anaerobic yield is the principal practical advantage of anaerobic treatment: very little sludge is produced, and the energy that would have gone into cells leaves as methane instead. • Solids retention time, also called sludge age or mean cell residence time, is the mass of solids in the system divided by the mass wasted per day: θc = V X / (Qw Xw + Qe Xe), and in the steady state 1/θc = Y U − kd. • SRT is the master design variable of every suspended growth system, and the reason is this: an organism whose maximum growth rate is less than 1/θc cannot replace the cells wasted and is washed out of the system.

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Nitrifiers grow slowly, so nitrification requires a long SRT — typically more than 5 days at 20 °C and more than 10 to 15 days at 10 °C — and a plant that nitrifies in summer may cease to do so in winter purely because of this. • A safety factor of 2 to 3 on the minimum or washout SRT is applied in design. • The food to microorganism ratio is the alternative and older loading parameter:

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F/M = Q S0 / (V X), in kg BOD per kg MLVSS per day.

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F/M and SRT are inversely related: a high F/M means a young, fast-growing, poorly settling sludge, and a low F/M means an old, well-oxidised, well-settling but possibly pin-flocking sludge. • Worked illustration.

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A reactor of 2000 m³ holds MLVSS of 2500 mg/L and receives 5000 m³/d at 200 mg/L BOD.

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F/M = (5000 × 200)/(2000 × 2500) = 1 000 000/5 000 000 = 0.2 kg BOD per kg MLVSS per day, which is a conventional activated sludge loading.

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If 50 m³/d of sludge at 10 000 mg/L is wasted, θc ≈ (2000 × 2500)/(50 × 10 000) = 10 days, ignoring the solids lost in the effluent.

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Aerobic, Anaerobic and Anoxic Processes Process Electron acceptor Products and characteristics Aerobic Dissolved oxygen Carbon dioxide, water, new cells.

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Fast, high yield, complete BOD removal, but large energy cost for aeration and much sludge Anoxic Nitrate (no dissolved oxygen) Nitrogen gas, carbon dioxide, new cells.

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Used for denitrification; recovers alkalinity and oxygen equivalent Anaerobic Carbon dioxide and sulphate (no oxygen or nitrate) Methane, carbon dioxide, hydrogen sulphide, few new cells.

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Low yield, net energy producer, but slow and temperature-sensitive • The distinction between anoxic and anaerobic is the single most frequently examined definition in the chapter.

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Anoxic means free of dissolved oxygen but containing nitrate, which serves as the electron acceptor; anaerobic means free of both oxygen and nitrate.

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Denitrification is therefore an anoxic process and not an anaerobic one, even though it occurs in a tank with no aeration. • Anaerobic digestion proceeds in four sequential steps, which must be known in order: hydrolysis of polymers to monomers; acidogenesis to volatile fatty acids, alcohols, hydrogen and carbon dioxide; acetogenesis to acetate, hydrogen and carbon dioxide; and methanogenesis to methane and carbon dioxide. • Hydrolysis is rate-limiting for particulate substrates, and methanogenesis is rate-limiting for soluble ones.

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The methanogens are the slow, sensitive step: they are strict anaerobes, grow slowly, and are inhibited below pH 6.5, so an overload causes acid to accumulate, the pH to fall, the methanogens to be further inhibited and the digester to sour — a self-reinforcing failure known as a stuck or sour digester. • The volatile acid to alkalinity ratio is the standard early warning of this condition and should be kept below about 0.3 to 0.4.

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Reactor Types and Growth Configurations • Batch reactor — filled, reacted and emptied; concentration falls with time.

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Simple, flexible and ideal for small or intermittent flows; the sequencing batch reactor is its modern form, carrying out fill, react, settle, decant and idle in one tank, which removes the need for a separate clarifier. • Completely mixed (CSTR) — the contents are uniform and equal to the effluent, so the organisms live at the low outlet substrate concentration throughout.

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Its great virtue is that an incoming shock or toxic load is immediately diluted by the whole tank volume, so it is robust against variable industrial flows.

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Its vice is that it needs a larger volume than plug flow for the same removal in a first-order reaction. • Plug flow — the fluid moves as successive slugs without longitudinal mixing, so the substrate concentration is high at the inlet and falls along the length.

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It achieves a given removal in a smaller volume than a CSTR for first-order kinetics, but it is vulnerable at the inlet end, where the high load demands the greatest oxygen supply and where a toxic slug strikes undiluted.

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Tapered aeration, supplying most air at the head of the tank, is the standard answer to the oxygen problem, and step feed, distributing the influent along the tank, is the answer to the load problem. • Real reactors lie between these ideals, and the dispersion number or tanks-in-series model is used to describe them; a tracer study reveals the actual pattern.

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Suspended growth Attached growth (biofilm) Biomass Free-floating flocs in the mixed liquor Fixed film on media, stone, plastic or carrier Examples Activated sludge, SBR, oxidation ditch, lagoons Trickling filter, RBC, MBBR, biotower, anaerobic filter Biomass control By wasting sludge;

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SRT directly controlled Self-regulating by sloughing;

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SRT not directly controlled Sludge return Essential; defines the process Not required Energy High; continuous aeration of the whole volume Lower; natural draught or low-power aeration Shock resistance Moderate; diluted by tank volume Good; the film is protected and recovers quickly Operator attention High;

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MLSS, SVI, return ratio, wasting Low; little to adjust Weakness Bulking, washout, high energy Ponding, filter flies, odour, poorer effluent from stone media

1

This section covers the activated sludge process and its modifications, trickling filters, moving bed biofilm reactors, stabilization ponds, lagoons and constructed wetlands, together with nutrient removal by nitrification and denitrification and by phosphorus removal, and the anaerobic technologies used for wastewater treatment and biogas recovery.

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The Activated Sludge Process • Settled sewage enters an aeration tank where it is mixed with returned activated sludge; the mixture, called mixed liquor, is aerated; it passes to a secondary clarifier where the biomass settles; most of the settled sludge is returned to the head of the aeration tank and the surplus is wasted. • Two points define the process and are regularly examined: the return of sludge is what allows the solids retention time to be set independently of the hydraulic retention time, and the wasting rate is what actually fixes the SRT.

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Without return, the biomass would simply wash out.

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Parameter Conventional Extended aeration High rate F/M, kg BOD/kg MLVSS·d 0.2 to 0.5 0.05 to 0.15 0.4 to 1.5 SRT, days 5 to 15 20 to 40 0.5 to 2 HRT, hours 4 to 8 18 to 36 1.5 to 3 MLSS, mg/L 1500 to 3000 3000 to 6000 200 to 1000 Return ratio 0.25 to 0.75 0.75 to 1.5 0.25 to 1.0 BOD removal 85 to 95 per cent 90 to 98 per cent 60 to 75 per cent Sludge produced Moderate Very little; well stabilised Large; unstable Nitrification Possible in warm weather Complete None • Sludge settleability is measured by the sludge volume index:

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SVI = (settled volume in mL/L after 30 minutes × 1000) / MLSS in mg/L, in mL/g.

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An SVI of 50 to 150 mL/g indicates good settling; above about 150 the sludge is bulking and will carry over the clarifier weir. • Worked illustration.

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MLSS 3000 mg/L settling to 300 mL/L:

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SVI = 300 × 1000 / 3000 = 100 mL/g, which is satisfactory. • The chief operating problems: filamentous bulking, cured by chlorinating the return sludge, raising the dissolved oxygen, correcting nutrients or adding a selector zone; rising sludge, caused by denitrification in the clarifier floating the settled blanket on nitrogen bubbles, cured by removing sludge faster; pin floc from an excessively long sludge age; and foaming from Nocardia or from surfactants. • The modifications and what each is for: step aeration distributes the influent along the tank to even out oxygen demand; contact stabilization aerates the return sludge separately and so needs a smaller total volume; the oxidation ditch is a looped channel with brush or disc aerators giving long SRT and simple operation; the sequencing batch reactor performs everything in one tank in timed phases and so needs no clarifier or return pumps; the membrane bioreactor replaces the clarifier with a membrane, allowing very high MLSS and an excellent effluent at high energy cost; and the pure oxygen system uses covered tanks and oxygen instead of air for strong wastes.

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Attached Growth Systems • The trickling filter is not a filter.

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It is a bed of rock or plastic media over which settled sewage is distributed by a rotating arm; the biofilm growing on the media absorbs and oxidises the organic matter, air passes through by natural draught, and the film periodically sloughs off and is removed in the secondary clarifier. • Rock media is 1.5 to 2.5 m deep with a specific surface of 40 to 70 m²/m³; plastic media allows 6 to 12 m depth with 80 to 200 m²/m³ and far better ventilation. • Low-rate filters use a hydraulic loading of 1 to 4 m³/m²·d with no recirculation, give 80 to 90 per cent BOD removal and nitrify; high-rate filters use 10 to 40 m³/m²·d with recirculation and give 65 to 85 per cent. • Recirculation serves several purposes at once: it maintains a continuous wetting of the media, dilutes a strong or toxic influent, returns organisms to seed the upper layers and increases the hydraulic shear that keeps the film thin and active. • The NRC formula is the standard design expression for a single-stage filter:

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E = 100 / [1 + 0.44 √(W / V F)], where W is the BOD load in kg/d, V the media volume in m³ and F the recirculation factor, F = (1 + R)/(1 + 0.1R)² with R the recirculation ratio. • Characteristic problems are ponding, where the surface clogs with excessive growth; filter flies (Psychoda), controlled by flooding or by maintaining the hydraulic flushing rate; odour, from anaerobic conditions in a clogged bed; and poor performance in cold weather. • The rotating biological contactor carries the film on closely spaced discs rotating slowly, about 40 per cent submerged, so that the film is alternately exposed to sewage and to air.

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It uses little energy and needs little attention, but shaft failure and media breakage have been persistent problems. • The moving bed biofilm reactor (MBBR) carries the film on small free-floating plastic carriers, kept in motion by aeration, which are retained in the tank by a sieve.

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It combines the compactness and shock resistance of attached growth with the simplicity of a single tank; there is no sludge return and no risk of bulking, the biomass is self-regulating, and the process can be retrofitted into an existing tank to uprate it.

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The IFAS arrangement adds carriers to a conventional activated sludge tank to do the same.

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Ponds, Lagoons and Constructed Wetlands Type Depth Retention Function Anaerobic pond 2.5 to 5 m 1 to 5 d Takes a very high organic load; settles and digests solids; removes 50 to 70 per cent of BOD; smells if overloaded or if sulphate is high Facultative pond 1.0 to 2.0 m 15 to 40 d Aerobic at the surface through algal photosynthesis and anaerobic at the bottom; the workhorse of the series Maturation pond 1.0 to 1.5 m 5 to 10 d each Removes pathogens by sunlight, high pH and long retention; polishes the effluent Aerated lagoon 2 to 5 m 3 to 10 d Mechanically aerated; much smaller area than a pond but needs power and a settling stage • The facultative pond works by symbiosis, and this is the key examinable idea: algae photosynthesise and release the oxygen that the aerobic bacteria use to oxidise the organic matter, while the bacteria release the carbon dioxide and nutrients that the algae use.

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Hence the pond must be shallow enough for light to reach a useful depth, and the effluent carries a high suspended solids load in the form of algae — which can cause an apparently treated effluent to fail a suspended solids standard. • Ponds are cheap, simple, robust, need no power and no skilled operator, and remove pathogens better than any conventional process; against this they need a great deal of land, perform poorly in cold weather, and may cause odour and mosquito nuisance.

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For small Nepali municipalities with land available they remain a serious option. • Constructed wetlands plant reeds, typically Phragmites or Typha, in a gravel bed.

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The horizontal subsurface flow wetland keeps the water below the gravel surface, which avoids odour and mosquitoes and is the common form; the vertical flow wetland is dosed intermittently and transfers far more oxygen, so it nitrifies; the free water surface type most resembles a natural marsh.

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Treatment is by physical filtration in the gravel, by the biofilm on the gravel and roots, and only to a small extent by uptake into the plants, a point commonly misunderstood.

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Typical area is 3 to 10 m² per person, and the chief failure mode is clogging of the inlet zone, which is why good primary treatment ahead of the bed is essential.

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Several Nepali institutions, notably hospitals in the Kathmandu Valley, use them successfully.

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Nutrient Removal • Nitrification is the two-step autotrophic oxidation of ammonia: ammonia to nitrite by Nitrosomonas, and nitrite to nitrate by Nitrobacter.

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The stoichiometry must be known:

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4.57 g of oxygen are required and 7.14 g of alkalinity as calcium carbonate are destroyed per gram of ammonia nitrogen oxidised.

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The alkalinity consumption is what causes the pH to fall in a poorly buffered nitrifying plant, and the optimum pH is 7.5 to 8.0.

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Nitrifiers are slow-growing, require a long SRT, need at least 2 mg/L of dissolved oxygen, and are strongly inhibited by low temperature and by many industrial chemicals. • Denitrification is the heterotrophic reduction of nitrate to nitrogen gas under anoxic conditions.

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It requires a carbon source, which is supplied either from the raw sewage, by placing the anoxic zone first and recycling nitrified mixed liquor to it — the Modified Ludzack-Ettinger arrangement — or by dosing methanol.

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It returns 3.57 g of alkalinity as calcium carbonate and recovers about 2.86 g of oxygen equivalent per gram of nitrate nitrogen reduced. • Placing the anoxic zone ahead of the aerobic zone, rather than after it, therefore achieves three things at once: it uses the free carbon of the raw sewage instead of purchased methanol, it recovers half the destroyed alkalinity, and it recovers part of the oxygen.

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This is why pre-denitrification is the standard arrangement. • Worked illustration.

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Removing 30 mg/L of ammonia nitrogen requires 30 × 4.57 = 137 mg/L of oxygen and destroys 30 × 7.14 = 214 mg/L of alkalinity as CaCO3.

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If the raw sewage has only 200 mg/L of alkalinity, nitrification will exhaust it and the pH will crash, so lime must be added or denitrification provided to return 30 × 3.57 = 107 mg/L. • Phosphorus removal is achieved chemically or biologically.

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Chemical precipitation with alum, ferric chloride or lime is simple and reliable but increases the sludge quantity considerably and consumes alkalinity.

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Enhanced biological phosphorus removal exploits the polyphosphate-accumulating organisms: an anaerobic zone is placed at the head of the plant, where the organisms release phosphate and store volatile fatty acids; on passing into the aerobic zone they take up phosphate far in excess of their growth requirement — luxury uptake — and the phosphorus leaves with the wasted sludge.

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The anaerobic contact zone is therefore essential and must not be allowed to receive nitrate, which would be consumed in preference and destroy the mechanism.

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Anaerobic Technologies and Biogas • The upflow anaerobic sludge blanket (UASB) reactor is the most important anaerobic process for municipal sewage in warm climates.

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Wastewater is distributed across the floor and flows upward through a blanket of dense granular sludge; the gas, liquid and solids are separated in a three-phase separator at the top, which retains the biomass within the reactor.

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The key to the process is the formation of settleable granules, which decouples the solids retention time from the hydraulic retention time and allows treatment at only 6 to 12 hours of HRT. • Advantages are very low sludge production, no aeration energy, a net production of biogas, a small footprint and low operating cost.

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Limitations are a performance that falls sharply below about 20 °C, an effluent that still requires aerobic polishing to meet a BOD standard, no nutrient removal, dissolved methane lost in the effluent, and odour from hydrogen sulphide where the sulphate content is high.

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These limitations are why UASB plants in Nepal and India are followed by polishing ponds. • Other configurations: the anaerobic filter, packed with media on which the film grows; the anaerobic baffled reactor, a series of compartments through which the flow passes up and down, which is robust and well suited to decentralised systems; and the expanded granular sludge bed, a high-rate development of the UASB. • Biogas yield.

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The theoretical yield is 0.35 m³ of methane per kilogram of COD stabilised at standard temperature and pressure, rising to about 0.40 m³ at 35 °C.

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Biogas is typically 60 to 70 per cent methane and 30 to 40 per cent carbon dioxide, with traces of hydrogen sulphide, and has a calorific value of about 20 to 25 MJ/m³, roughly 60 per cent that of natural gas. • Worked illustration.

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A UASB treating 2000 m³/d of sewage at 400 mg/L COD and removing 75 per cent stabilises 2000 × 400 × 0.75 /1000 = 600 kg COD/d, giving 600 × 0.35 = 210 m³ of methane per day, or about 300 m³/d of biogas at 70 per cent methane — enough to generate roughly 400 to 500 kWh per day.

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This section covers decentralized wastewater treatment by septic tanks and soak pits, and the handling of sludge by gravity thickening, dewatering and stabilization, including anaerobic digestion, composting, land application and incineration.

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Decentralized Systems • Decentralized treatment handles the wastewater at or near where it is generated, instead of conveying it to a central works.

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It is appropriate where the settlement is scattered, where the terrain makes a sewer network expensive, where the effluent is to be reused locally, and where institutional capacity for a large plant is lacking — which covers much of Nepal outside the main valley towns. • The septic tank is the basic unit.

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It is a watertight, covered, buried tank in which the sewage is held quietly so that solids settle to form sludge, grease and light solids rise to form scum, and the settled solids are digested anaerobically over long periods.

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It is a settling and digestion tank, not a treatment plant: it removes 60 to 70 per cent of suspended solids and only 30 to 50 per cent of BOD, and its effluent is anaerobic, odorous and full of pathogens, so it must always be followed by a soak pit, a leach field or another dispersal or treatment unit. • Design rules: liquid detention of 24 to 48 hours at the design flow, with 1 to 3 days for small installations; length to width ratio of 2:1 to 4:1; liquid depth not less than 1.0 m, usually 1.2 to 1.8 m; free board of about 0.3 m; sludge accumulation allowance of about 30 L per person per year; two compartments, the first about two-thirds of the volume, which greatly improves the effluent; inlet and outlet tees or baffles dipping below the liquid surface to retain the scum; and desludging every two to three years. • Never disinfect or wash out a septic tank completely, because the digesting sludge is the seed for continued operation.

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A small residue is deliberately left after desludging. • The soak pit (soakaway) is a lined or unlined pit filled with coarse aggregate which receives the septic tank effluent and allows it to percolate into the soil.

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Its size is fixed by the percolation rate of the soil, measured by a percolation test, and it must be at least 15 m, and preferably 30 m, from any well, and at least 2 m above the highest groundwater table.

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Clogging of the infiltration surface by a biological mat is the normal failure mode, which is why a duplicate pit used alternately is good practice. • Other decentralized units: the Imhoff tank, with settling above and a separate digestion chamber below so that rising gas does not disturb the settling; the anaerobic baffled reactor, a series of up-and-down compartments that forces the flow through the sludge; and the DEWATS package, which combines a settler, a baffled reactor, an anaerobic filter and a planted gravel filter and is widely used in Nepal and the region.

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Sludge Quantities and Thickening • Sludge is the real cost of wastewater treatment: it is a small fraction of the flow but may absorb half the capital and operating cost of the plant. • Primary sludge is 4 to 8 per cent solids, grey, greasy and highly putrescible; waste activated sludge is only 0.6 to 1.2 per cent solids, brown, flocculent and difficult to dewater; digested sludge is 3 to 6 per cent, dark and inoffensive. • The volume of sludge is governed overwhelmingly by its water content, and this is the arithmetic that must be mastered:

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V1/V2 = P2/P1, where P is the percentage of solids. • Worked illustration.

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Thickening a sludge from 1 per cent to 4 per cent solids reduces its volume to one-quarter.

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Dewatering it further from 4 per cent to 20 per cent reduces it to one-fifth of that, that is to one-twentieth of the original.

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Each step removes only water, yet the saving in tankage, transport and disposal is enormous — which is why thickening is the first operation in every sludge train. • Gravity thickening is a settling tank with a picket-fence stirrer whose slow rotation opens channels and releases trapped water.

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It is designed on solids loading rate, 90 to 150 kg/m²·d for primary sludge but only 20 to 40 kg/m²·d for waste activated sludge, which thickens poorly.

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Dissolved air flotation and centrifugal thickening are used for activated sludge instead, and gravity belt thickeners are now common. • Conditioning with a polyelectrolyte, or formerly with lime and ferric chloride, is applied before mechanical dewatering to coagulate the fine solids and release bound water.

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Stabilization Method Conditions Comment Anaerobic digestion Mesophilic 30 to 38 °C, 15 to 30 d; thermophilic 50 to 57 °C, 10 to 15 d Reduces volatile solids 45 to 60 per cent, destroys pathogens, produces biogas; the standard method for medium and large plants Aerobic digestion Ambient, 10 to 20 d, continuous aeration Simple, no gas, low capital cost but high power cost; suits small plants and waste activated sludge Lime stabilization Raise pH above 12 for 2 h Rapid, simple, kills pathogens and stops odour, but adds mass and does not reduce volatile solids Composting Aerobic, 55 to 65 °C for 3 d or more Produces a usable soil conditioner; needs a bulking agent such as sawdust, straw or wood chips Heat treatment 150 to 200 °C under pressure Conditions as well as stabilises; energy intensive • Anaerobic digestion is the most important of these and is examined most often.

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It proceeds through the four steps given in section 9.4; mesophilic digestion at about 35 °C with 15 to 30 days of retention is the usual choice; and the temperature must be held steady, since the methanogens tolerate fluctuation badly. • A standard-rate digester is unheated and unmixed, stratifies into scum, supernatant, digesting sludge and grit, and needs 30 to 60 days; a high-rate digester is heated and completely mixed, needs only 15 to 20 days, and is followed by a second-stage tank for settling and gas storage. • Gas production is about 0.75 to 1.1 m³ per kilogram of volatile solids destroyed, giving 15 to 22 L per person per day; the gas is 60 to 70 per cent methane, and in a well-run plant it covers the digester heating and a useful part of the plant's power. • Worked illustration.

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A plant produces 1000 kg/d of dry solids of which 70 per cent are volatile and 50 per cent of the volatile solids are destroyed: destroyed VS = 1000 × 0.70 × 0.50 = 350 kg/d, giving about 350 × 0.9 = 315 m³/d of biogas, roughly 220 m³ of methane with an energy content of some 7 to 8 GJ per day.

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Dewatering, Disposal and Reuse Method Cake solids Characteristics Sludge drying beds 25 to 45 per cent Cheapest; no power or skilled labour; needs much land and dry weather; the natural choice in Nepal outside the monsoon Belt filter press 18 to 25 per cent Continuous, moderate power, needs polymer conditioning; simple to operate Centrifuge 20 to 30 per cent Compact and enclosed, so no odour; high power and maintenance; needs polymer Plate and frame filter press 30 to 45 per cent Highest cake solids; batch operation, high labour, heavy conditioning needed Drying lagoons 20 to 40 per cent Very cheap for digested sludge where land is available • A sludge drying bed is 200 to 300 mm of graded sand over gravel with underdrains; water is removed partly by drainage in the first days and partly by evaporation thereafter; the cake is lifted by hand or by a small loader.

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It requires about 0.1 to 0.25 m² per person and only works in dry weather, which limits it to the non-monsoon months unless it is covered. • Land application of stabilised sludge is the preferred disposal route where it can be practised, because the sludge supplies nitrogen, phosphorus and organic matter to the soil.

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The constraints are the pathogen content, which sets the degree of stabilisation required and the crops on which it may be used; the heavy metal content, which sets a cumulative loading limit and is the usual reason for refusing industrially contaminated sludge; the nitrogen loading, which must not exceed the crop uptake if groundwater is not to be polluted by nitrate; and public acceptance. • Composting with a bulking agent produces a stable, friable, marketable product and destroys pathogens in the thermophilic phase, but needs space, bulking material and odour control. • Incineration achieves the greatest volume reduction, destroys all pathogens and organic contaminants, and is the route for sludge too contaminated for land, but it is the most expensive option, requires auxiliary fuel unless the sludge is well dewatered, and produces an ash containing the concentrated heavy metals together with flue gas requiring cleaning. • Landfilling of sludge is the last resort, requiring the sludge to pass a free-liquids test and to be co-disposed or used as a daily cover material. • Faecal sludge management deserves separate mention for Nepal, where most urban households use septic tanks: the sludge removed by tankers must be received at a designated faecal sludge treatment plant, typically using settling and thickening tanks, planted or unplanted drying beds and a polishing pond, rather than being discharged to a river or a field, which remains common practice.