Winter Operation of a Biological Plant: Temperature, Sludge Age and Bulking

Winter is a different plant, not the same plant with colder water

Cold weather changes three inputs at once: the water arrives colder, the pollutant load arrives higher, and the biology works more slowly. The last of those is the dangerous one, because it is invisible. A plant that looked well designed in September can drift out of compliance in January without any single obvious failure, and by the time the ammonia number moves, the nitrifying population has already been reduced.

What follows is the operating discipline that keeps a plant inside its limits through the cold period, drawn from the way influent patterns and unit behaviour actually change between seasons.

Manage the whole train, not just the biology

Every unit has to keep doing its job, and the units that matter most in winter are the ones whose failure quietly shifts load downstream: screens, grit chambers, hydrolysis-acidification tanks and sludge dewatering machines. A blinded screen or a dewatering machine returning solids to the head of the works adds load to a biological stage that has no capacity to spare. Most episodes of unstable effluent can be traced to a failure somewhere upstream of the aeration tank rather than inside it.

Process behaviour, sludge characteristics and degradation patterns should be reviewed against the daily and monthly variation in influent quality and quantity. The aim is to treat the full flow, stay compliant, and still cut energy and chemical cost — which is only possible if the operating parameters are matched to the actual water rather than to the design sheet.

Protect nitrogen removal first

Nitrogen passes through amination, nitrification and denitrification before leaving as gas. The optimum for nitrification is 20-30 °C; below 15 °C the rate falls and at 5 °C it stops altogether. Denitrification works best between 20-40 °C, and below 15 °C both the growth rate and the metabolic rate of the denitrifiers decline. In the north-east of China, winter wastewater sits at around 10 °C or lower, well under the optimum for either group.

Nitrifiers are more sensitive to cold than denitrifiers, so nitrification tends to be the incomplete step, and careless operation in cold weather produces unstable ammonia almost immediately. Three levers compensate: raise the mixed liquor suspended solids, extend the sludge age into the 15-25 day range, and increase aeration somewhat. More air does two jobs — it holds dissolved oxygen up and it helps keep the tank warm, which is why it is the most widely used winter measure.

The essential point is that nitrifiers recover slowly. A shock load that would be absorbed in summer can cost weeks in January, so protection of the population matters more than correction after the fact.

Sludge bulking is a winter problem

Low temperature lowers sludge activity and makes the whole process easier to upset, which is why winter is the peak season for bulking. Three features stand out. It is very common: about 60% of municipal plants experience bulking every year. It is universal: every activated sludge configuration is affected, including intermittent aeration tanks that are otherwise the least prone. And the consequences are severe: sludge is lost, effluent suspended solids exceed the limit and capacity falls away.

Once bulking starts it is difficult to control and slow to reverse, so the first defence is prevention through an appropriate sludge load — not too low. Plants with an anaerobic or selector zone can use the biological selection effect to suppress filamentous organisms. Sludge condition should be tracked continuously, and an SVI above 150 treated as a signal rather than a curiosity.

Chemical options exist and each has a price. 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 work in some cases and increase sludge production sharply, which complicates downstream handling. Adding soil or fibrous material suits certain industrial waters such as paper-mill effluent, but that is a short-term measure only. Chlorine and hydrogen peroxide have both been used successfully against filamentous growth; chlorine is cheap and easy to apply on site, which is why more than half of all plants use it. The target is the filaments attached to the surface of the floc-formers, but the two groups do not differ much in chlorine sensitivity, so the dose has to be tuned — an overdose damages the flocs as surely as the filaments.

Dose chemicals as a final safeguard, not as the process

Where a plant has a high-efficiency sedimentation tank or a chemical stage, the biological system should be optimised first and the chemical stage used to catch what escapes. Relying on chemicals to hold quality steady generates large volumes of chemical sludge, and if that sludge cannot be handled the plant enters a loop it cannot exit. Dosing needs a written procedure, a named person responsible, and a dose that is adjusted rather than left at the commissioning value.

Tighten influent control when flows fall

Winter flows are lower, and the industrial share of the influent rises as a result. That shift is the reason source control matters more in the cold months. When online influent data look abnormal, verify on site at once; if a high-pollutant discharge is confirmed, take emergency measures, retain evidence, and notify the responsible authority promptly, in writing where necessary.

Data, dewatering and the parts nobody enjoys

Statistical analysis of operating data deserves particular emphasis, and its conclusions should drive the adjustments. Laboratory results must be timely, accurate and reliable, and production data must be equally sound, because an analysis built on bad numbers is worse than no analysis.

Weak winter sludge makes dewatering harder and cake moisture harder to control. Thickening and dewatering should be managed closely, with machine scheduling matched to production, 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 practice is worth revisiting: jar tests establish the pattern, and make-up with tap water reduces polymer demand. Cold make-up water seriously hinders the dissolution of polyacrylamide, so heating can be fitted to the make-up tank, the make-up line and the water tank.

Inspection, frost and instruments

At low temperature, outdoor structures are prone to frost heave and ice formation, so inspection rounds should cover process and equipment across every unit, with attention to the inlet, the outlet and the biological tanks. Once snow and frost arrive, maintenance moves indoors, so key equipment should be serviced in advance — aeration and sludge removal above all. A key unit that fails in winter costs far more to repair than one serviced beforehand, and it takes the plant with it.

Online instruments matter as much. Influent COD and ammonia nitrogen analysers, and process dissolved oxygen and pH probes, must be kept working so that data are acquired and uploaded accurately and the monitoring system can catch deviations while they are still small. An instrument that has drifted is more dangerous than one that has failed, because it reports confidently and wrongly.