咨询服务电话
热门新闻
Sewage Treatment Plant Installation, Commissioning And Troubleshooting Guide
A sewage treatment plant is judged on the quality of the water it discharges, but that result is produced by the whole facility rather than by one unit. Wastewater arrives with a variable flow and load, is screened and grit-separated, is then treated biologically or chemically, and finally is disinfected before it leaves the site. Every stage assumes the previous stage has done its job.
This guide is written for the person responsible for getting the plant built, started and kept working: it covers the principles that determine performance, what happens during installation and commissioning, how to set the process running correctly, and how to diagnose the failures that occur in practice. It is deliberately practical, because most problems on a treatment plant are not exotic and are usually traceable to a small number of controllable parameters.
The treatment train is a sequence of unit operations, each removing a different fraction of the load. Inlet works and pumping come first, then preliminary treatment, then the biological stage where most of the organic matter is removed, then secondary clarification, and finally tertiary polishing and disinfection. A typical sequence for a municipal plant serving a population of tens of thousands looks like this.
| Stage | Typical equipment | Primary purpose | Key operating indicator |
|---|---|---|---|
| Inlet works | Pump station, inlet control | Equalise flow and lift the wastewater | Level and flow |
| Screening | Bar screen, rotary screen | Remove rags and large debris | Differential head across screen |
| Grit removal | Grit channel with classifier | Remove sand, gravel and heavy inert material | Grit depth and pump run time |
| Primary treatment | Primary clarifier | Remove settleable solids and floatables | Blanket depth and effluent quality |
| Biological treatment | Activated sludge, SBR, MBBR or MBR | Remove dissolved organic matter and nutrients | Dissolved oxygen, mixed liquor solids, sludge age |
| Secondary clarification | Secondary clarifier | Separate biomass from treated water | Blanket depth and turbidity |
| Tertiary treatment | Filtration, chemical dosing | Polish residual solids and nutrients | Filtrate turbidity and head loss |
| Disinfection | Chlorination or ultraviolet reactor | Inactivate pathogens | Dose or dose rate, residual |
The biological stage is where most treatment capacity is created, and it works because microorganisms consume the dissolved organic matter that the plant is required to remove. The active biomass is held in the aeration basin as a suspension of floc, and the amount of this biomass is described by the mixed liquor suspended solids. The rate at which it is fed with organic material is the food to microorganism ratio, and the time it is retained in the system is the sludge age.
These two parameters, mixed liquor suspended solids and sludge age, govern almost everything about a conventional activated sludge plant. Mixed liquor suspended solids below roughly 2000 milligrams per litre usually means the plant has less biological capacity than the load requires, and the result is untreated organic matter in the effluent. Excessively high values mean the sludge is being held too long, the tank fills with biomass, and the clarifier struggles.
Dissolved oxygen is the third control point. Oxygen must be supplied at the rate the organisms consume it, which is set by the organic load. Too little oxygen limits treatment and produces poorly settling sludge, because the bacteria do not form the dense floc that settles well. Too much oxygen wastes the largest operating cost of the plant, and in some cases breaks up floc and carries fine solids into the effluent.
Installation quality determines how much commissioning effort is needed afterwards. Most delays and cost overruns on treatment projects come from decisions made before the tank is on the ground: setting out errors, inadequate wall thickness, poor concrete finish in a clarifier floor, or a pipe route that leaves nowhere to put a valve.
Before construction starts, the drawings should be checked against the site rather than only against the schedule. Levels of incoming and outgoing pipework have to be reconciled with the hydraulic profile, because a single mismatch in a invert level can force a pump to run outside its efficient range or prevent a tank from draining at all. Bends in sludge and scum lines need to be long-radius and gentle, because sharp elbows in sludge service are the most common cause of repeated blockage.
During installation, three items deserve close supervision. First, the internal surfaces of clarifiers and aeration basins must be smooth and free of laitance, because uneven surfaces produce turbulence and interfere with sludge collection. Second, pipework must be flushed and inspected before it is connected to the tanks, and the tank must be hosed down before the biological culture is introduced.
Mechanical equipment on a treatment plant is straightforward in principle but demanding in detail. Pumps must be installed with proper suction conditions, which for most wastewater pumps means a flooded suction, an adequate straight run and a non-return valve in the correct position. Screws that are not supported, or a suction pipe that runs uphill for no reason, produce vibration and cavitation that appear months later as mysterious wear.
Electrically, the arrangement of the control panel and the power distribution is where commissioning problems usually surface. A motor that starts in the wrong sequence, a variable speed drive that is not configured for the duty, or a control signal that is scaled incorrectly will not be discovered until the first week of operation. The table below lists the items to verify before the plant is energised.
| Check item | Verification method | Typical defect found |
|---|---|---|
| Motor rotation | Jog each motor and confirm direction | Reversed rotation on a pump |
| Phase and voltage | Measure at the motor terminals | Voltage drop across a long cable run |
| Variable speed drives | Run the full speed range and check response | Wrong ramp or minimum speed limit |
| Level instruments | Compare reading against a manual dip | Calibration offset from the tank bottom |
| Flow instruments | Check reading against a bucket or tank volume test | Pulse scaling error |
| Dissolved oxygen probes | Check against a wet process probe | Slow response and membrane fouling |
| Back-up power | Simulate a supply failure | Generator not switching to essential loads |
| Alarm annunciation | Trigger each alarm from the field | Alarms not mapped to the control room |
Commissioning is a sequence, not a single event. The plant is first filled with clean water to confirm that the tanks hold water, that the pipework is valved in the right direction and that nothing leaks. Mechanical equipment is then run individually on clean water, which removes the risk of introducing the biological culture into a system with a hidden mechanical fault. Only after that does the biological culture enter the plant.
The first loading is the delicate part. Seeding is carried out with activated sludge from a similar working plant, dosed into the aeration basin at a rate that gives a usable mixed liquor concentration without shocking the organisms. Air is then introduced gradually, and dissolved oxygen is held at the set point before any significant organic load is admitted. The first days are run at reduced flow, and the effluent is watched closely even though the permit limits may not yet apply.
A practical commissioning schedule over the first month follows a logical ramp. In the first week the objective is a stable biomass and consistent settling. In the second week the flow is raised towards the design average while the sludge age is adjusted. In the third week the tertiary and disinfection stages are brought on line and their performance is checked against laboratory results.
Once the plant is running, the work becomes a matter of holding several variables in balance, and the balance changes with the season and with the load. The table below gives the starting point for the main control parameters and the direction in which to move them when performance drifts.
| Control parameter | Typical starting range | Move up when | Move down when |
|---|---|---|---|
| Mixed liquor suspended solids | 2500 to 4000 mg per litre | Effluent organic load is rising | Clarifier blanket is deepening |
| Sludge age | 10 to 20 days | Nitrification is incomplete | Biological floc is pinpoint |
| Dissolved oxygen | 1.5 to 2.5 mg per litre | Organic removal is poor | Energy use is excessive |
| Food to microorganism ratio | 0.15 to 0.35 per day | Nitrification is wanted | Bulking or foaming appears |
| Return sludge ratio | 0.5 to 1.0 of flow | Biomass is washing out | Blanket is rising in the clarifier |
| Waste sludge ratio | 2 to 5 percent of flow | Blanket is too deep | Solids are washing out |
Changes should be made one variable at a time and then given time to show an effect. The biological process responds over days, so several simultaneous adjustments produce a result that cannot be attributed and a plant that drifts rather than settles.
Failures on a treatment plant repeat, and the symptom usually points to a limited number of causes. The skill is in reading the pattern rather than in testing every possibility. The table below links the most frequent symptoms to their usual causes and to the first checks to make.
| Symptom | Most likely causes | First checks |
|---|---|---|
| Cloudy effluent with stable organic results | Clarifier short-circuiting, blanket carryover, filter breakthrough | Blanket depth, weir level, filter head loss |
| Rising effluent ammonia | Loss of sludge age, temperature drop, insufficient aeration | Sludge age calculation, dissolved oxygen, blower output |
| Poor settling, pinpoint floc | Low mixed liquor solids, over-aeration, detergent ingress | Microscope or settle test, mixed liquor reading |
| Sludge rising in the clarifier | Insufficient sludge wasting, hydraulic overload | Wasting rate, flow split, blanket depth |
| Odour at the inlet works | Stagnant channels, blocked screens, long retention in grit chamber | Screen condition, cleaning frequency, channel flow |
| Foam on the aeration basin | Oil or surfactant in the influent, wasting too much sludge | Source inspection, sludge rate, grease load |
| Blower vibration or temperature rise | Poor inlet filter, worn impeller, incorrect operating point | Filter condition, machine condition, inlet pressure |
| Pump trips on start-up | Air lock, closed suction valve, blocked line | Suction line condition and valve positions |
A treatment plant degrades gradually, so the maintenance plan matters more than any single repair. Daily routines cover the inspection of screens, grit pumps, blowers and clarifier surfaces, together with the recording of flow, dissolved oxygen, mixed liquor suspended solids, sludge volume and alarm history. The value of these records is that they turn a developing problem into a visible trend before it becomes an exceedance.
Preventive maintenance schedules should address the equipment whose failure stops the plant: screen rake chains, drive gearboxes, pump mechanical seals, blower bearings and filters, diffuser membranes and control system modules. Spares should be matched to the actual failure history of that plant, with the long lead time items held on site rather than a general inventory kept for everything.
Training protects performance when experienced staff are absent. New operators need supervised instruction in the process, the control system, the mechanical equipment, laboratory methods and site safety, followed by a period of shadowing on shift, with competency recorded against specific tasks rather than simply a course attendance certificate.
A sewage treatment plant delivers its intended result only when the whole train is understood as one system: the screens protect the pumps, the grit chamber protects the biological stage, the aeration basin creates the capacity, the clarifier separates it and the disinfection stage makes the discharge safe. Installing well means checking levels, hydraulics, access and controls before concrete is poured, because the cheapest moment to correct a design error is on the drawing.
