Winter Operation of Wastewater Treatment Plants: Keeping Effluent Compliant

Winter operation has its own difficulties: the cold period is long, water temperature is low, influent pollutant concentrations are higher and sludge activity is weaker. Running a plant through the winter therefore demands stronger operational management so that the adverse conditions are offset, the plant runs efficiently, effluent stays within limits and the required reduction in pollutant load is delivered. The practices below summarise experience gained from influent conditions and winter operation in previous years.

1. Manage the Whole Process, Not Just the Biology

Every treatment unit must be kept working as it should, starting from the details. Faults and problems should be detected, analysed and solved promptly, so that a small problem does not grow into a large one that destabilises the whole system. Particular attention goes to units whose poor operation throws extra load onto the biological stage — screens, grit chambers, hydrolysis-acidification tanks and sludge dewatering machines — because that is one of the commonest routes to unstable effluent.

The plant's own process behaviour, sludge characteristics and pollutant degradation patterns, together with the daily and monthly variation in influent quality and quantity, should all feed into process optimisation: treat the full flow, keep effluent compliant, and at the same time save energy and reduce operating cost.

2. Adjust Operating Parameters

In winter the influent is generally more concentrated, the water is colder, the activated sludge is less active and reactions proceed more slowly. Operating parameters must therefore be adjusted to suit the plant's process and its actual influent characteristics.

3. Protect Nitrogen Removal

In biological nitrogen removal, nitrogenous compounds pass through amination, nitrification and denitrification in turn and finally leave the water as nitrogen gas. The optimum temperature for nitrification is 20–30 °C; below 15 °C the nitrification rate falls, and at 5 °C it stops altogether. Denitrification works best at 20–40 °C; below 15 °C both the growth rate and the metabolic rate of the denitrifiers decline. In north-east China winter wastewater sits at around 10 °C or lower, far below the optimum for either group, and nitrogen removal efficiency suffers accordingly.

Nitrifiers are more sensitive to low temperature than denitrifiers, so nitrification tends to be incomplete, and careless operation in cold weather very easily produces unstable ammonia nitrogen. The countermeasures are to raise the mixed liquor suspended solids, to extend the sludge age (15–25 days is a sensible range), and to increase aeration somewhat, which both helps hold water temperature and raises dissolved oxygen — a widely used way of keeping ammonia nitrogen under control. The key to ammonia removal is the nitrifying population, so the system must be kept stable and protected from severe shock loads: in winter nitrifiers recover only with difficulty.

4. Control Sludge Bulking

Because sludge activity drops and operation is easily upset at low temperature, winter is a peak season for sludge bulking. Three features stand out. First, it is very common: some 60% of municipal plants experience bulking every year. Second, it is universal: every type of activated sludge process is affected, even intermittent aeration tanks that are otherwise the least prone. Third, the consequences are severe: sludge is lost, effluent suspended solids exceed the limit and treatment capacity falls sharply.

Once bulking starts it is hard to control and slow to reverse. The first defence is to hold an appropriate sludge load — not too low. Plants with an anaerobic or selector zone can exploit the biological selection effect to suppress filamentous bacteria. Operators should track sludge condition continuously and take SVI above 150 seriously, using chemical control if necessary. Synthetic high-molecular-weight cationic polymers control bulking well with little effect on sludge production, but they are expensive. Inorganic coagulants such as lime or ferric chloride also work in some cases, at the cost of a large increase in sludge production that complicates downstream sludge handling. Adding soil or fibrous material suits certain industrial wastewaters such as paper-mill effluent, but that is only a short-term measure.

Chlorine and hydrogen peroxide have both been used successfully to suppress filamentous growth. Chlorine is relatively cheap and easy to apply on site, so it is widely used: more than half of all plants use it against filamentous bulking. The aim is to kill the filaments attached to the surface of the floc-forming organisms, but the two groups do not differ much in chlorine sensitivity, so the dose must be tuned to kill filaments without harming the flocs; an overdose is just as harmful to sludge quality.

5. Dose Chemicals Sensibly

Where a plant has a high-efficiency sedimentation tank or a chemical treatment stage, the biological system should always be optimised first and the chemical stage used as a final safeguard. Relying on chemicals to hold water quality steady produces large quantities of chemical sludge, and if that sludge cannot be handled the system falls into a vicious circle. Dosing must follow a defined procedure and responsibility, with a nominated person in charge, and the dose must be adjusted to avoid waste.

6. Tighten Control of the Influent

Winter flows are lower and the industrial share of the influent tends to rise, so source control matters more. When online influent data look abnormal, operators should verify on site at once; if a high-pollutant discharge is confirmed, emergency measures should be taken, evidence retained, and the responsible authority informed promptly, in writing if necessary.

7. Collect, Organise, Compile and Analyse Production Data

Statistical analysis of operating data deserves particular emphasis, and its conclusions should drive adjustments in operation. Laboratories must deliver timely, accurate and reliable analytical results, and production data must be equally dependable — reliable data are the precondition for any analysis, and if the precondition is wrong the conclusions will be too.

8. Strengthen Sludge Dewatering

Weak winter sludge makes the dewatering system harder to run and the cake moisture content harder to control. Thickening and dewatering should be managed closely and the dewatering machines scheduled to suit production needs, so that the biological system keeps an appropriate MLSS. Above all, a malfunctioning dewatering machine must not be allowed to send waste or chemical sludge round in circles and raise the solids load on the biological stage. Flocculant use should be explored actively: jar tests can establish the pattern, and using tap water for make-up reduces the polymer dose. Because cold make-up water seriously hinders the dissolution of polyacrylamide in winter, heating devices can be fitted to the make-up tank, the make-up water line and the water tank.

9. Keep Inspection Safe

At low temperature, outdoor structures are prone to frost heave and ice formation, so inspection rounds should cover both process and equipment across every unit, with particular attention to the inlet, the outlet and the biological tanks, so that abnormalities are found and dealt with in good time.

10. Maintain Equipment and Instruments

Once snow and frost arrive, maintenance work moves indoors, so key equipment should be serviced in advance — above all the aeration and sludge removal systems. A key unit that fails in winter costs far more to repair and causes far more damage than one serviced beforehand. Online instruments (influent COD and ammonia nitrogen, and process DO and pH) must also be kept working, so that data are acquired and uploaded accurately and the monitoring system can do its job of catching and correcting deviations early.