Problems identified in a treatment works, and the solutions applied to them. A companion to The Science of Waste Water Treatment, written from the operator's side: what is seen, what it means, what is done, and the green, amber and red criteria that say whether it worked.
Randhir Rawatlal · WASH R&D Centre, UKZN · draft of 2 September 2026 for discussion with Mohamed Dildar
The science book explains why a works has the shape it has. This book is about the days when the works does not behave. A process controller arriving on shift is not asked to explain nitrification. They are asked why the clarifier is carrying a blanket at a metre from the surface, whether the tanker queued at the gate should be allowed to discharge, and what to do about the blower that tripped at two in the morning. Treatment in practice is the recognition of a limited number of recurring problems and the disciplined application of known responses to them.
1Walk and readings→2Flag against band→3Diagnose the cause→4Act now, then fix→5Verify: green→6Record
Every entry has the same parts, and the parts are the method. A problem is first a symptom, something seen on the walk or read from an instrument. The symptom has a cause, and there are usually two or three candidates that must be told apart before anything is done. The response has two halves: act now, which protects the effluent and the licence today, and fix, which stops the problem recurring. The criteria strip states, in the colours of a control chart, what green looks like, what amber looks like, and what red looks like, with the time recovery should take. A response without a criterion is a guess.
GreenIn band. Nothing to do beyond the record.
AmberDrifting. Open the entry, diagnose, act before the shift ends.
RedOut of band or the licence at risk. Act now, then fix.
Each entry carries a code, the compartment letter followed by a number, so that a logbook line or a maintenance request can cite it, and a priority tier that sets the order in which competing problems are attended to on a bad day.
Tier 1 · Safety and environmentChlorine gas, confined space, raw sewage bypass to the river, digester gas. Attend before anything else.
Tier 2 · Licence complianceAny condition that will put a final effluent sample outside the licence limits within the week.
Tier 3 · Process stabilityDrifts in sludge age, DO, MLSS and blanket that will become a Tier 2 problem if not corrected.
Tier 4 · Cost and conditionEnergy, chemical, wear and maintenance backlog. Attended to on a planned basis.
2Works map
The entries placed on the works, in the order of the site walk. Each code is coloured by its tier. Click a code to open the entry.
Most problems are caught first by a reading leaving its band, and a reading only means something at a named compartment. The strips below are the bands for a nitrifying extended-aeration works in KwaZulu-Natal, drawn as the control chart the operator should keep. They should be replaced by the site's own once six months of data exist. A reading in amber or red is a flag, and a flag is an obligation to open one of the entries listed beside it.
Inflow rateInlet flume or meter · Continuous, logged hourly
Two readings are missing from most logbooks and matter more than most of the ones present: the oxygen uptake rate of the mixed liquor, which is the first thing to fall under a toxic load, and the microscope. A working microscope at the plant, and one person able to distinguish filaments, free-swimming ciliates and stalked ciliates, shortens the diagnosis in half the aeration entries from days to minutes.
4Inlet works
The inlet works is cheap insurance for everything downstream, and its failures are almost always mechanical or procedural rather than biological. The three that do the most damage are rags, grit and uncontrolled tanker discharges. Science book, ch. 3.
I1
Screens blinding and the bypass channel running
Tier 2 · Licence
Symptom
Level upstream of the screen rising against the bypass weir. Rags wound on the aerator shaft or the RAS pump impeller. Screenings skip not filling although flow is normal.
Cause
Rake mechanism stalled or its trip not reset, raking interval set too long for the rag load, or bar spacing too wide so that rags pass and accumulate downstream. In older works the screen is simply undersized for what the catchment now sends.
Act now
Manual rake, reset and test the mechanism, confirm the level switch.
Fix
Raking on level differential rather than on a timer, a manual raking round on every shift written into the shift sheet, and a fine screen (6 mm) where the catchment includes informal settlement or a hospital.
GreenUpstream level below the bypass sill on every shift; screenings 0.01 – 0.03 m³ per 1 000 m³ treated
AmberLevel within 100 mm of the sill, or screenings volume falling while flow is steady
RedBypass weir flowing, or rags on the aerator at the weekly inspection
RecoverySame shift
I2
Grit passing the grit channel
Tier 3 · Process
Symptom
Grit banks in the corners of the aeration basin and in the anoxic zone, pump volutes worn through in months rather than years, digester effective volume falling year on year.
Cause
Channel velocity outside the design 0.3 m/s, usually too high at peak flow because the second channel is out of service or the proportional weir is damaged. Grit classifier or air-lift off. In aerated grit chambers, air rate set for odour rather than for grit.
Act now
Bring both channels into service, clear the classifier.
Fix
Check the velocity at three flows with a float and a stopwatch, restore the control weir, put the classifier on the daily walk.
GreenGrit removed 0.02 – 0.05 m³ per 1 000 m³; none in a settled sample from the basin floor drain
AmberGrit removal below 0.02 m³ per 1 000 m³ with normal flow
RedGrit visible in the basin, or a pump volute worn through inside a year
RecoveryWeeks; the basin banks stay until drained
I3
Tanker discharges and shock loads
Tier 2 · Licence
Symptom
DO in the aeration basin falling to zero within an hour and staying there, pH dropping below 6.5, a sharp odour at the inlet, effluent ammonia rising over the following two days. Often traced to a queue of tankers discharging between 10:00 and 14:00.
Cause
Faecal sludge from pit and septic tank emptying is fifty to two hundred times the strength of sewage in COD and ammonia. A 10 m³ load discharged in fifteen minutes into a 5 ML/d works is the equivalent of an hour's flow arriving at once. Industrial loads disguised as domestic are the worse case.
Act now
Stop discharges until DO recovers, and refuse any load without a source declaration.
Fix
A tanker register with time, source, volume and a pH and conductivity dip on every load, a discharge point that feeds a holding tank bled to the inlet at night, and a written maximum daily volume set from the aeration capacity.
GreenDO above 1 mg/L through the discharge window; register reconciles with the gate log
AmberDO dips below 1 mg/L for under an hour on a discharge day
RedDO at zero for over an hour, or effluent ammonia up the day after a discharge day
RecoveryDO within hours; ammonia 3 – 5 d
Science book, ch. 3, by sewer and by tanker
I4
Wet-weather hydraulic overload
Tier 2 · Licence
Symptom
Inflow at three to five times average dry-weather flow within an hour of rain. Clarifier blanket rising to the weirs, solids over the weirs, MLSS falling by a third in a day, effluent suspended solids and COD failing for a week after the storm.
Cause
Stormwater ingress and infiltration into the sewer network: illegal roof and yard connections, collapsed pipes, open manholes. This is a network problem that presents at the works, and the works cannot fix it. What the works can do is stop losing its biomass.
Act now
Raise RAS to the pump maximum, stop wasting, switch to step-feed or contact-stabilisation mode if the basin allows, and use the storm bypass only as the licence permits and with the event logged.
Fix
Report inflow peaks with dates to the network section as a standing monthly item, and size a storm tank if the pattern is chronic.
GreenPeak under 2.5 × ADWF; blanket below the mid-depth alarm through the storm
AmberPeak 2.5 – 3 × ADWF, MLSS down under 15 % after the event
RedSolids over the weir, MLSS down over 30 %, or an unlogged bypass
RecoveryMLSS back in band within 5 d
I5
Fats, oils and grease
Tier 3 · Process
Symptom
A grey scum blanket on the anoxic zone and clarifier, grease balls at the weirs, stable brown foam on the aeration basin, inlet pipework losing capacity.
Cause
Restaurants, abattoirs, dairies and food processors on the catchment discharging without grease traps or with traps never emptied. Grease selects for Nocardia and Microthrix and feeds the foaming problem in A4.
Act now
Skim and remove scum to the digester or to disposal daily, never return it to the basin.
Fix
Trade-effluent inspection of the named dischargers, grease-trap servicing as a bylaw condition, and a scum baffle on the clarifier if it has none.
GreenNo stable foam on the basin; scum removed per week trending down over the quarter
AmberScum on the clarifier surface on more than two days a week
RedStable brown foam on the basin, or grease balls at the weirs
RecoveryWeeks, once the source is controlled
I6
Flow measurement absent or uncalibrated
Tier 3 · Process
Symptom
Flow on the logbook is a constant, or the flume is silted, or the meter reads the same at midnight as at midday. Every loading calculation, sludge age and chlorine dose then rests on a guess.
Cause
Silt in the approach channel of the flume, ultrasonic head not verified against a staff gauge, magnetic flow meter offline for years and its display taken as true.
Act now
Clean the flume, check the ultrasonic against the staff gauge at three levels, log the correction.
Fix
A monthly verification written into the maintenance calendar, and a stand-by method (weir plate or drawdown test on a wet well) so that a meter failure does not mean a blank column.
GreenDiurnal pattern visible, night minimum under 50 % of day peak; monthly verification within ± 5 %
AmberVerification 5 – 10 % out, or over a month since the last one
RedFlat trace, or no verification on record
RecoveryA day to verify; a month to rebuild the record
5Primary settling
Primary tanks, where a works has them, are the cheapest treatment on site and the most often neglected, because their failure mode is quiet. A primary tank does its work through desludging, and a tank that is not desludged becomes a digester of the wrong kind. Science book, ch. 3, primary settling.
P1
Septic primary tank
Tier 3 · Process
Symptom
Black effluent from the primary tank with a hydrogen sulphide smell, gas bubbles lifting lumps of sludge to the surface, primary effluent BOD higher than the raw sewage on a bad day, downstream DO demand rising.
Cause
Sludge held in the hopper for days rather than hours: desludging pump failed, desludging done once a day by habit, or the digester feed line blocked so that the operator stops drawing.
Act now
Desludge until the draw runs thin, then set a draw on every shift.
Fix
Timed automatic desludging of a few minutes every hour, pump on the critical spares list (W4), and a sample tap on the draw line.
GreenPrimary effluent grey; draw at 3 – 6 % DS; BOD 30 – 40 % below raw
AmberDraw thinner than 3 % or BOD removal under 25 %
RedBlack effluent, sulphide smell, sludge lifting
Recovery1 – 2 d
P2
Short-circuiting and scum carry-over
Tier 4 · Cost
Symptom
Solids removal well below the expected 50 to 60 %, a visible current from inlet to weir on the surface, scum passing under a damaged baffle.
Cause
Inlet stilling well broken or missing, weirs out of level by more than a few millimetres so that one side takes most of the flow, scum board rusted through, scraper off.
Act now
Restore the scraper and skim the scum by hand.
Fix
Level the weirs with a surveyor's level and replace the baffle: drained-tank jobs, planned for a dry month.
GreenSuspended solids removal above 50 % on a composite sample
AmberRemoval 40 – 50 %
RedRemoval under 40 %, or scum passing the baffle
RecoveryAfter the drained-tank repair
6Aeration basin
The aeration basin holds the biomass, and the biomass is the works. Every problem in this chapter is a problem of one of four quantities: oxygen, food-to-mass ratio, sludge age and the ions the bacteria need. Most bad days are one of these quantities having drifted out of band without anyone noticing, and the microscope and the settled-volume cylinder are how they are noticed. Science book, ch. 2 and 4.
A1
Low dissolved oxygen
Tier 2 · Licence
Symptom
DO below 0.5 mg/L at the end of the aerobic zone, mixed liquor going grey then black, a sour smell at the basin, effluent ammonia rising over three days, filamentous bulking following within two weeks.
Cause
In order of likelihood: an aerator or blower down, diffusers fouled, MLSS allowed to climb until the oxygen demand exceeds supply, an organic overload from I3 or I5, or the DO probe itself fouled and reading low. Tell them apart in that order: the probe check is thirty seconds and is done first.
Act now
Verify the probe against a portable meter, restore aeration, cut MLSS by wasting if it is above band, refuse tanker loads.
Fix
Stand-by blower or aerator, diffuser cleaning on the maintenance calendar, and a DO setpoint that is actually connected to the blower.
GreenDO 1.5 – 2.5 mg/L at the end of the aerobic zone
AmberDO 0.5 – 1.5 mg/L for more than a shift
RedDO under 0.5 mg/L, or mixed liquor grey to black
RecoveryDO within a shift; ammonia 3 – 5 d
A2
Over-aeration
Tier 4 · Cost
Symptom
DO above 4 mg/L through the day and night, energy bill out of proportion to the load, pin floc and a turbid supernatant in the settling test, nitrate in the effluent higher than it should be because the anoxic zone is receiving oxygen.
Cause
Aerators on a fixed timer or on full time, no DO control, or a control loop with the setpoint left at commissioning value while the load has fallen.
Act now
Switch off one aerator overnight and watch the DO log.
Fix
A DO control loop with a 2 mg/L setpoint, or where no control exists, a night-time aerator timer tuned against the DO log. Aeration is 50 to 60 % of the works' electricity, so this is usually the largest single saving available.
GreenDO within 1.5 – 2.5 mg/L through the diurnal cycle; kWh per kg COD removed falling
AmberDO 2.5 – 4 mg/L at night
RedDO above 4 mg/L day and night
RecoveryImmediate on the setpoint; a month to see it in the bill
A3
Filamentous bulking
Tier 2 · Licence
Symptom
SVI above 150 mL/g and rising, a slowly settling, fluffy sludge that leaves a clear supernatant, clarifier blanket climbing week by week until solids go over the weir. Under the microscope, filaments bridging between flocs.
Cause
The filament names the cause. Type 021N and Thiothrix: septic feed or sulphide from I3 and P1. Sphaerotilus and type 1701: low DO (A1). Microthrix parvicella: long sludge age, low F/M, grease (I5), and cold months. Nocardia: grease and long sludge age, with foam (A4). Nutrient deficiency, which shows as type 021N with an industrial feed, is rare on a domestic works.
Act now
Raise RAS and lower the blanket. Chlorinate the RAS at 2 – 5 g Cl₂ per kg MLSS per day only as a last resort and only while the cause is being removed.
Fix
The cause from the list. Where the cause is low F/M, an anoxic or anaerobic selector at the head of the basin is the durable fix.
GreenSVI 80 – 120 mL/g; filament count falling on the weekly slide
AmberSVI 120 – 150 mL/g
RedSVI above 150 mL/g, or blanket rising week on week
RecoveryTwo sludge ages, 30 – 40 d
A4
Foaming
Tier 3 · Process
Symptom
Two foams, told apart by colour and stability. A thick, stable, brown, greasy foam that persists on the clarifier and the digester is Nocardia or Microthrix. A white, billowing foam that collapses when hosed is a young sludge after a washout or a surfactant load.
Cause
Brown foam: grease (I5), long sludge age, warm months. White foam: sludge age too short after I4 or A8, or a detergent discharge.
Act now
Brown: remove the foam physically and never return it, since the organisms float and re-seed. Spraying foam back into the basin is the most common wrong response. White: stop wasting until the MLSS recovers.
Fix
Brown: cut sludge age and remove the grease at source. White: the trade-effluent register for the surfactant source.
GreenFoam under 10 % of the basin surface; none on the digester
AmberFoam 10 – 30 % of the surface, collapsing when hosed
RedStable brown foam over 30 %, or foam on the digester
RecoveryBrown, one sludge age; white, days
A5
Pin floc and turbid supernatant
Tier 3 · Process
Symptom
SVI low, often below 70 mL/g, a fast-settling sludge but a cloudy supernatant carrying fine particles, effluent suspended solids 20 – 40 mg/L although the blanket is low. Under the microscope, small dense flocs with few filaments and almost no free-swimming life.
Cause
An old sludge: sludge age well above 25 days because wasting is not being done (A8), often combined with over-aeration (A2) shearing the flocs.
Act now
Start wasting today at the programme volume.
Fix
Sludge age held at 15 – 20 d by the wasting programme, and aeration cut to band.
GreenSupernatant clear; effluent SS under 15 mg/L; stalked ciliates on the slide
AmberEffluent SS 15 – 25 mg/L with a low blanket
RedEffluent SS above 25 mg/L with SVI under 70
RecoveryOne sludge age
A6
Nitrification failure
Tier 2 · Licence
Symptom
Effluent ammonia climbing past 3 and then 6 mg/L as N, nitrate falling to near zero at the same time, aeration basin pH drifting down.
Cause
The nitrifiers are the slowest organisms in the works and the first to be lost. Four causes, checked in order: sludge age below 10 d after a washout or over-wasting; DO below 1 mg/L (A1); alkalinity consumed, since nitrification uses 7.1 mg of alkalinity as CaCO₃ per mg of ammonia-N and a soft KwaZulu-Natal water can run out; a toxic load (A10). Winter lengthens the recovery from all four.
Act now
Stop wasting, restore DO, dose lime or soda ash to hold alkalinity above 80 mg/L.
Fix
Sludge age held by a wasting programme (A8), and alkalinity on the daily sheet.
GreenEffluent ammonia under 3 mg/L N; alkalinity above 80 mg/L
RedAmmonia above 6 mg/L (licence), or alkalinity under 50
RecoveryOne to three sludge ages, longer in July
Science book, ch. 4, the nitrogen relay
A7
Denitrification failure
Tier 3 · Process
Symptom
Effluent nitrate above 10 mg/L as N while ammonia is fine, and the clarifier showing the rising-sludge problem in C4.
Cause
The anoxic zone is receiving oxygen (A2, or the mixed-liquor recycle pumping DO back), the recycle pump is off so that nitrate never reaches the anoxic zone, or the anoxic zone is starved of carbon because primary settling is removing too much or the feed is weak.
Act now
Start the a-recycle at 3 – 5 × inflow and trim aeration at the end of the aerobic zone so that the recycle carries under 1 mg/L DO.
Fix
If carbon is the limit, bypass a fraction of raw sewage around the primary tank.
GreenEffluent nitrate under 10 mg/L N; no rising sludge
AmberNitrate 10 – 15 mg/L
RedNitrate above 15 mg/L (licence), or clumps rising in the clarifier
RecoveryDays once the recycle runs
A8
Sludge age not controlled
Tier 3 · Process
Symptom
MLSS drifting up over months to 6 000 mg/L and beyond, then crashing after a storm, SVI wandering, and the wasting valve opened when the clarifier looks bad and closed when it looks good. Nobody on site can say what the sludge age is.
Cause
Wasting done by eye rather than by calculation. Sludge age is the one number the operator controls that sets everything else, and it is set by the volume wasted per day.
Act now
Calculate today's sludge age from MLSS, basin volume and yesterday's WAS.
Fix
A wasting programme: target sludge age 15 – 20 d for a nitrifying works, daily WAS volume = basin mass ÷ (sludge age × WAS concentration), wasted at the same time every day and written down. Adjust the target, never the day's volume, in response to SVI and effluent ammonia.
GreenMLSS within ± 10 % of target across a month; sludge age on the daily sheet
AmberMLSS 10 – 20 % off target, or sludge age 10 – 15 d or 20 – 25 d
RedMLSS outside 2 000 – 6 000 mg/L, or sludge age unknown
RecoveryOne sludge age to settle
A9
Phosphorus not removed
Tier 2 · Licence
Symptom
Effluent ortho-phosphate above the licence limit, on a works designed for biological phosphorus removal or fitted with chemical dosing.
Cause
Biological: nitrate reaching the anaerobic zone through the RAS (A7 not solved) and consuming the volatile fatty acids the phosphate-accumulating organisms need, or the anaerobic zone aerated by a leaking valve or surface turbulence. Chemical: ferric or alum dosing pump off, tank empty, or dose set once and never re-tuned to the load. Secondary release from sludge left too long in the clarifier or in a thickener that returns liquor.
Act now
Chemical: restart dosing from the day's phosphate reading. Biological: confirm the anaerobic-zone ORP is below −150 mV.
Fix
Biological: fix A7 first. Chemical: dose set at a molar ratio of 1.5 – 2 metal to phosphorus with a supply contract that does not run out. Both: keep sludge moving and avoid returning thickener liquor at peak flow.
GreenEffluent ortho-P under 5 mg/L on the weekly composite
AmberOrtho-P 5 – 10 mg/L
RedOrtho-P above the licence limit
RecoveryChemical, a day; biological, one sludge age
A10
Toxic shock
Tier 2 · Licence
Symptom
Oxygen uptake rate collapsing within hours, DO rising with no change in aeration, the sludge deflocculating into a milky supernatant, nitrification stopping the same day, a chemical smell at the inlet, sometimes a colour.
Cause
An industrial discharge, a tanker load of the wrong thing, a chlorine or disinfectant dump from a swimming pool or a hospital, or heavy metals from a plating shop. The source is found by walking the sewer upstream of the works on the day, not the week after.
Act now
Divert to a storm tank if there is one, stop RAS chlorination if any, stop wasting, and hold a sample of the inflow for the lab and the trade-effluent officer. Re-seed with mixed liquor from a neighbouring works if the sludge is dead.
Fix
Trade-effluent permits with sampling, and a relationship with the industrial dischargers such that they phone before they discharge.
GreenOxygen uptake rate 20 – 40 mg O₂/L·h
AmberOUR down by a third in a day
RedOUR under 10 mg O₂/L·h, or milky supernatant
RecoveryDays to weeks; nitrification per A6
7Secondary clarifier
The clarifier is where the works is judged, because the effluent is what leaves it. A clarifier does two jobs, clarification of the overflow and thickening of the underflow, and most of its problems are the second job failing and taking the first with it. Science book, ch. 5.
C1
High sludge blanket and solids washout
Tier 2 · Licence
Symptom
Blanket within a metre of the surface, flocs visible over the weir, effluent suspended solids above 25 mg/L, MLSS falling.
Cause
Four candidates: bulking sludge that will not thicken (A3), RAS pump down or throttled so that sludge is not being returned, hydraulic overload (I4), or a scraper stopped so that sludge is not reaching the hopper (C2). The blanket profile tells them apart: a bulking blanket is uniform and slowly rising, a RAS failure shows a thick blanket with a clear settled zone above it, an overload comes and goes with the flow.
Act now
Raise RAS, confirm the pump is actually moving sludge by measuring the RAS concentration, stop wasting.
Fix
Open the entry that matches the blanket profile.
GreenBlanket under 0.5 m from the floor at all three points; effluent SS under 25 mg/L
AmberBlanket 0.5 m to 30 % of depth, rising
RedBlanket above 30 % of depth, or solids over the weir
RecoveryHours for RAS; weeks if A3
C2
Scraper or bridge stopped
Tier 3 · Process
Symptom
Sludge accumulating on the far side of the tank, gas bubbles and black clumps rising from that side, the hopper running thin while the blanket is high.
Cause
Drive motor tripped, drive wheel slipping on a wet or algae-covered track, torque switch tripped on a grit bank, or the bridge deliberately stopped and forgotten.
Act now
Reset and inspect. A tank that has stood for more than a day is drained and cleaned before restarting, or the septic sludge returns to the basin and takes the DO with it.
Fix
Bridge rotation on the shift walk, track cleaned weekly, drive on the critical spares list.
GreenBridge rotation logged each shift; blanket uniform across three points
AmberBridge stopped under a day, blanket uneven
RedBridge stopped over a day, or black clumps rising
RecoverySame day, or after a drained clean
C3
Uneven and fouled weirs
Tier 4 · Cost
Symptom
Flow over one quarter of the weir length and none over the rest, algae mats on the weir plates and launder, effluent SS higher than the blanket would suggest.
Cause
Weir plates out of level, launder settled, algae growth encouraged by sunlight and a nutrient-rich effluent.
Act now
Brush the weirs.
Fix
Weirs brushed weekly, levelled annually, and covered where algae is chronic.
GreenFlow visible over the full weir length; effluent SS within 5 mg/L of the cylinder supernatant
AmberFlow over under three quarters of the length
RedFlow over under half the length, or algae mats
RecoverySame day
C4
Rising sludge
Tier 3 · Process
Symptom
Clumps of sludge the size of a hand floating to the surface with small gas bubbles attached, a sludge that settles well in the cylinder and then lifts after an hour.
Cause
Denitrification in the clarifier: nitrate-rich sludge held in the blanket long enough to run out of oxygen, so that the bacteria reduce nitrate to nitrogen gas, which lifts the floc. The clarifier is doing the anoxic zone's job (A7).
Act now
Raise RAS and keep the blanket low so that sludge spends less time in the clarifier.
GreenNo lifting in the cylinder at 60 min; effluent nitrate under 10 mg/L
AmberLifting at 60 min in the cylinder, none in the tank
RedClumps on the clarifier surface
RecoveryDays
8Disinfection and final effluent
Disinfection is the last barrier and the one the regulator tests most directly. Its problems are rarely biological. They are supply, dosing equipment and contact time, and they are made worse by every upstream problem that puts solids into the contact tank. Science book, ch. 6, chemistry with a shield problem.
D1
Chlorine supply and dosing failure
Tier 1 · Safety
Symptom
Zero free residual at the contact tank outlet, E. coli in the final effluent in the thousands per 100 mL, and on the walk an empty HTH drum, a dosing pump running dry, or a gas cylinder at zero with no changeover.
Cause
Procurement lead time on chlorine (W4), a dosing pump diaphragm failed, a gas changeover manifold not working, or the chlorinator ejector water supply lost when a pump tripped. Gas chlorine adds a safety condition: a leak is Tier 1 in its own right, and the works must have a working leak detector, breathing apparatus and a trained person on every shift.
Act now
Restore dose, by manual hypochlorite dosing if need be, and log the period without disinfection.
Fix
A minimum stock of four weeks' supply as a standing order, a stand-by dosing pump, and a gas changeover manifold tested monthly.
GreenFree residual 0.5 – 1.0 mg/L at the outlet every two hours; E. coli within limit weekly
AmberResidual 0.1 – 0.5 mg/L, or under two weeks of stock
RedResidual zero, or a gas leak alarm
RecoveryHours
D2
Contact tank short-circuiting and sludge in the tank
Tier 2 · Licence
Symptom
Residual at the dosing point fine, residual at the outlet fine, E. coli still failing. Sludge deposits visible in the tank corners, or a dye put in at the inlet appearing at the outlet in ten minutes in a tank designed for thirty.
Cause
Baffles missing or collapsed so that the contact time is a fraction of design, solids from C1 settled in the tank and shielding the organisms, or chlorine demand from those solids consuming the dose before it reaches the bacteria.
Act now
Drain and clean the tank.
Fix
Restore the baffles, and fix C1 so that the effluent entering carries under 25 mg/L SS.
GreenDye test minimum contact above 20 min at peak flow; E. coli within limit four weeks running
AmberContact 15 – 20 min, or sludge visible in the corners
RedContact under 15 min, or E. coli failing with residual present
RecoveryAfter the clean
D3
Sampling and laboratory practice
Tier 2 · Licence
Symptom
Compliance results that contradict the plant's own daily kit readings, results arriving three weeks after the sample so that nothing can be done about them, a sample point in a stagnant corner, samples left in the sun before collection.
Cause
Sampling is a measurement, and an uncontrolled measurement is a random number. The regulator's judgement rests on this number.
Act now
Move the sample point to flowing effluent and chill the next sample within the hour.
Fix
A written sampling procedure: named point, named time, chilled within the hour, chain-of-custody form, laboratory turnaround as a contract term. Daily kit readings for ammonia, nitrate, phosphate and residual chlorine, so that the monthly lab result is a confirmation rather than a surprise.
GreenKit and lab agree within 20 %; lab result in hand within 7 d
AmberAgreement 20 – 40 %, or turnaround 7 – 14 d
RedKit and lab contradict, or turnaround over 14 d
RecoveryOne sampling cycle
9Sludge line
Half the works is the sludge line, and it is where the neglected problems accumulate physically. A primary tank not desludged, a digester not fed, drying beds not lifted: each is a stock of solids somewhere on site that will eventually be dealt with, usually on the worst possible day. Science book, ch. 7.
S1
Digester souring
Tier 2 · Licence
Symptom
Digester pH falling from 7.2 toward 6.5 over a week, VFA to alkalinity ratio rising past 0.3 and then 0.5, gas production falling and the flame going out, hydrogen sulphide in the gas, a grey rather than black digested sludge.
Cause
The acid-formers have outrun the methanogens. Causes in order: heating lost and temperature falling more than a degree a day, an overfeed after a period of underfeeding, a toxic load in the feed (A10, or grease from I5), and grit and scum filling the tank so that the real retention time is half the nominal (S5).
Act now
Stop feeding, restore heating and mixing, dose sodium bicarbonate to bring alkalinity above 2 000 mg/L as CaCO₃. Resume feeding at a quarter rate when gas production returns and step up over two weeks.
Fix
A feed programme with a daily volume, and the temperature on the shift sheet.
GreenpH 6.8 – 7.4; VFA : alkalinity under 0.3; gas 0.8 – 1.0 m³ per kg VS destroyed
AmberpH 6.5 – 6.8, or VFA : alkalinity 0.3 – 0.5
RedpH under 6.5, VFA : alkalinity over 0.5, or flame out
Recovery2 – 4 weeks
Science book, ch. 7, digestion
S2
Digester foaming
Tier 3 · Process
Symptom
Foam in the gas line and pressure relief, sludge in the gas holder, a sudden fall in gas pressure.
Cause
Nocardia or Microthrix carried in with the waste sludge from A4, a shock feed, or over-mixing.
Act now
Reduce feed and mixing, clear the foam trap on the gas line.
Belt press cake below 12 % dry solids, filtrate black with solids, polymer consumption above 8 kg per tonne, sludge running off the belt edges.
Cause
Polymer dose or make-up wrong (the make-up concentration and ageing time matter as much as the dose), belt tension and wash water off, or the feed sludge changed because the digester is sour (S1) or the waste sludge is bulking (A3). A bulking sludge is a poorly dewatering sludge.
Act now
A jar test on the day's sludge to reset the dose; belt wash and tension checked.
Fix
Jar test at the start of every run, and the upstream entry if the sludge itself is the problem.
RedCake under 12 %, black filtrate, or polymer over 8 kg/t
RecoverySame run
S4
Drying beds waterlogged and unlifted
Tier 3 · Process
Symptom
Every bed occupied, none dry enough to lift, sludge standing in water a week after application, sand black and blinded, weeds in the beds.
Cause
Beds applied too deep (more than 200 mm), applied before the previous cake was lifted, sand not replaced so that the drainage layer is blinded, underdrains blocked, and in the wet season, no cover.
Act now
Lift every bed that has reached 25 % DS and rod the underdrains.
Fix
A bed programme: application depth 150 – 200 mm, lift at 25 % DS or after a set number of days, scrape and top up sand after each lift, rod the underdrains quarterly. Where the works has both beds and a press, the beds are the dry-season route and the press the wet.
GreenA quarter of the beds empty and dry at any time; cycle under 21 d in the dry season
AmberUnder a quarter empty, or cycle 21 – 30 d
RedNo bed available, or sludge standing in water after a week
RecoveryOne bed cycle
S5
Grit and scum accumulation in the digester
Tier 4 · Cost
Symptom
Gas yield per kilogram of feed falling year on year, souring (S1) recurring at feed rates that used to be fine, a scum mat under the cover.
Cause
I2 and I5 over a decade. The real retention time is the nominal time multiplied by the fraction of the tank that is still liquid.
Act now
Sound the tank to estimate the grit depth.
Fix
A clean-out in a dry month with the second digester carrying the load, and I2 and I5 fixed so that the clean-out lasts.
Stockpiles of cake on site growing for months, leachate running back to the inlet or off site, no offtake contract in place, beds and press stopped because there is nowhere to put the product.
Cause
Disposal is treated as someone else's problem. The Sludge Guidelines classify the product by microbial, stability and pollutant class, and the class determines the lawful routes: agricultural use, composting, landfill. A works that has not classified its sludge cannot contract its disposal.
Act now
Return stockpile leachate to the inlet at night, never to the stormwater, and load out to the nearest lawful route.
Fix
Classify the sludge against the Guidelines, a disposal contract matched to the class, and a maximum stockpile that triggers a load-out.
GreenStockpile below the trigger volume; tonnes out per month equal to tonnes produced
AmberStockpile at 70 – 100 % of trigger
RedStockpile over trigger, leachate off site, or no classification
RecoveryMonths to contract
10Works-wide conditions
The entries in this chapter are not process problems, and they cause more effluent failures than all the process problems combined. A works with a competent controller, a stand-by blower and a month of chlorine in store will pass. A works without them will fail on the first bad week, however good its biology.
W1
Power outages and load shedding
Tier 1 · Safety
Symptom
Aeration off for two to four hours several times a day, DO at zero for those hours, inlet wet well overflowing to the bypass when the inlet pumps stop, chlorine dosing stopping with the ejector water.
Cause
Grid supply, and a generator that is either absent, undersized for the whole works, out of diesel, or not on automatic changeover.
Act now
Start the generator on the essential-load list, in order: inlet pumps, RAS, one aerator or blower, chlorination, digester mixing. Biomass survives four hours without air. It does not survive a clarifier that has been anaerobic overnight.
Fix
A generator sized to the essential-load list rather than to the whole works, a diesel stock for 72 hours, a changeover tested weekly, and a restart sequence on the wall of the control room.
GreenGenerator start logged on every outage; DO recovered within an hour of supply returning
AmberDO recovery over an hour, or diesel under 24 h
RedA bypass event caused by an outage, or no generator start
RecoveryHours
W2
Theft and vandalism
Tier 2 · Licence
Symptom
Cable stripped from the aerator supply overnight, aluminium weir plates and copper earthing gone, a pump removed from a wet well, the fence cut in the same place three weekends running.
Cause
An unattended site with resaleable metal and no consequences.
Act now
Report every incident with a case number so that the pattern is on record.
Fix
Armoured cable in concrete ducting, steel in place of aluminium and copper where the function allows, a night presence. The cost of the loss is the effluent failure, and that belongs in the budget motivation.
GreenIncidents per quarter falling; no effluent failure attributable to theft in the month
AmberOne incident in the month without effluent effect
RedAn effluent failure caused by theft
RecoveryPer incident
W3
Staffing, shift handover and the logbook
Tier 2 · Licence
Symptom
Nobody on site at night, one process controller for three works, a logbook with the same numbers every day, a handover that is a set of keys. The works classification under the regulations calls for a controller of a given class on shift, and the class on site is two below it.
Cause
Posts vacant, training not funded, and the works not classified so that the shortfall is invisible.
Act now
A shift sheet with the readings of chapter 2 and a place to write what was done about each flag, and a handover that walks the site together.
Fix
Classify the works, state the controller class and headcount that the classification requires, and put the gap on record with the municipality as the standing risk it is.
GreenReadings vary day to day; every flagged reading has an entry code and an action next to it
AmberFlags without actions on more than one sheet a week
RedSame numbers every day, or no controller on shift
RecoverySheets, a week; posts, a budget cycle
W4
Spares and procurement lead time
Tier 3 · Process
Symptom
A blower down for four months waiting for a bearing, chlorine ordered when the last drum is opened, a single RAS pump with no stand-by and a nine-week delivery.
Cause
Procurement designed for stationery, applied to a plant that runs twenty-four hours a day.
Act now
Write the critical spares list: every item whose failure stops aeration, sludge return, desludging or disinfection.
Fix
For each listed item, either a stand-by unit installed or a spare on the shelf. A standing order for chlorine and polymer at a minimum stock level. A framework contract with a mechanical contractor for the repairs the works cannot do.
GreenEvery critical item duplicated or in store; longest critical outage in the quarter under 7 d
AmberOne critical item neither duplicated nor in store
RedA critical item down over 7 d
RecoveryA procurement cycle
W5
Instruments unmaintained
Tier 3 · Process
Symptom
A DO probe reading 8 mg/L in a black basin, a pH meter that has not been calibrated in a year, an online ammonia analyser switched off because it kept alarming.
Cause
Instruments bought with the works and never given a maintenance line. A fouled DO probe is worse than none, because it is believed.
Act now
Verify every online reading against a portable meter today; a reading that contradicts the eyes is checked before it is acted on (A1).
Fix
A weekly clean and a monthly two-point calibration for every probe, with the log kept.
GreenCalibration log complete; online and portable within 10 %
AmberOnline and portable 10 – 25 % apart, or calibration over a month old
RedOver 25 % apart, or an analyser switched off
RecoveryA day
W6
Data recorded and never read
Tier 3 · Process
Symptom
Months of daily readings in a logbook, none plotted, the drift in A8 or A6 visible in hindsight to anyone who draws the graph and invisible to everyone who did not.
Cause
The logbook is treated as a record for the regulator rather than as an instrument for the operator.
Act now
Plot the last three months of the chapter 2 readings with their bands.
Fix
The bands drawn on a trend for each reading, updated daily, on the wall or on a screen, with the flags of the week reviewed at a short Monday meeting that assigns an entry code and an owner to each. This is the single practice that turns the rest of the book from a reference into a routine.
GreenEvery out-of-band reading has an entry code and an owner within 24 h
AmberFlags reviewed weekly but not owned
RedReadings not plotted
RecoveryA week to start
Science book, ch. 9, from flags to diagnoses
W7
Odour complaints
Tier 4 · Cost
Symptom
Complaints from the neighbouring settlement, worst in the early morning and in summer.
Cause
Odour is the symptom of another entry: septic feed (I3), a septic primary tank (P1), a low-DO basin (A1), a stopped clarifier (C2), sludge stockpiles (S6). It is diagnosed by walking the site at the time of the complaint and finding which compartment smells.
Act now
Walk the site at the hour of the complaint and name the compartment.
Fix
The upstream entry. Masking agents and covers treat the complaint and not the cause.
GreenNo H₂S above 1 ppm at the fence; complaints per month falling
AmberOne complaint in the month, source found
RedRepeated complaints, or H₂S over 1 ppm at the fence
RecoveryWith the upstream entry
11Symptom index
The book is used from this table on a bad day. Find what is seen, and open the entries in the order listed. The order is the order of likelihood on a domestic works, and the first check is usually the cheapest.
The licence for the site is the authority, and it is read before this table is. The General and Special limits of the General Authorisation are given as the common reference, since most works are licensed against one of them, and the Green Drop assessment scores against the licence. The daily bands of chapter 3 are set tighter than the licence on purpose, so that the flag is raised before the failure.
Limits above are the 2013 General Authorisation values and are a reference for the draft. The site licence, and any Green Drop condition attached to it, replaces this table in the published book.
13Outstanding items
This draft was written from the literature and from works visited, and its value depends on being corrected against a real plant. Three things are asked of the first reading.
Entries that are missing. The problems a works actually spends its time on that are not in this list, with the symptom as it is seen on the day. Trickling-filter works (ponding, filter flies, snails) and pond systems are absent from this draft and need their own chapters if the works in question use them.
Bands and criteria that are wrong for the site. Every number in chapter 3 and every green, amber and red line is a typical value and should be replaced by the site's own once its data have been plotted (W6).
The order of the symptom index. The order is a guess at likelihood and the first reader will know better.
Once corrected, the draft becomes a webbook alongside the science book, with the same illustrated treatment for each compartment, and the two are read together: the science for why, the practice for what to do. A later step, if the works want it, is a problem register in which each shift's flags are logged against these entry codes, so that the recurrence of a problem, and the time it took to clear, are on record for the works and for the Green Drop file.