Four Common Secondary Clarifier Abnormalities: Causes and Remedies

In day-to-day operation of a wastewater treatment plant, the secondary clarifier is where abnormalities show up most often. Rising suspended solids in the effluent, dissolved oxygen that is too low, sludge floating on the surface, and black sludge rising in blocks are problems most operators have met. This article analyses four common abnormalities and the measures used to correct them.

1. Suspended Solids in the Clarifier Effluent Increase

(1) Sludge bulking worsens the settling properties of the activated sludge, the sludge blanket rises close to the water surface and fragments of sludge escape over the effluent weir. The remedy is to analyse the causes of bulking and eliminate them one by one.

(2) A sudden increase in inflow raises the surface hydraulic loading of the clarifier, which increases the upward velocity and disturbs normal settling, so the flow carries sludge fragments over the weir. The remedy is to use the equalisation tank properly so that the inflow is as even as possible.

(3) The mixed liquor suspended solids in the aeration tank are too high, again bringing the sludge blanket close to the surface so that fragments escape over the weir. The remedy is to increase the waste activated sludge flow.

(4) Sludge disintegration reduces or destroys the flocculating ability of the activated sludge, and fragments leave with the flow. The remedy is to identify the cause of disintegration and deal with it.

(5) Poor performance of the sludge scraper or suction collector causes short-circuiting of sludge or flow inside the clarifier, so part of the sludge is not returned in time and stays too long in the tank, where it becomes anoxic, decomposes and escapes with the flow. The remedy is to repair the collector promptly and restore normal operation.

(6) The sludge simply remains too long in the clarifier, becomes anoxic and decomposes, and then escapes with the flow. The remedy is to increase the return sludge flow and shorten the retention time in the clarifier.

(7) When the water temperature is high and nitrate is abundant, denitrification occurs inside the clarifier; nitrogen gas lifts lumps of sludge to the surface and they leave with the flow. The remedy is again to increase the return sludge flow and shorten the sludge retention time.

2. Dissolved Oxygen in the Clarifier Effluent Is Too Low or Too High

(1) Sludge retained too long in the clarifier keeps consuming oxygen through aerobic micro-organisms, so dissolved oxygen in the effluent falls. The remedy is to increase the return sludge flow and shorten the retention time.

(2) A poorly performing scraper or suction collector leaves part of the sludge unreturned and over-retained, with the same result. The remedy is to repair the collector.

(3) A sudden rise in water temperature increases the oxygen demand of aerobic microbial activity and strengthens anaerobic activity in local anoxic zones, again lowering effluent dissolved oxygen. The remedy is to extend the retention time in equalisation and pre-treatment units, to recirculate the hot water through the equalisation tank, or to promote evaporation and cooling by stronger pre-aeration.

(4) If the organic loading on the aeration tank is low or the aeration capacity is excessive, effluent dissolved oxygen will be high while water quality remains good. The remedy is to draw more water from the equalisation tank to raise the loading, or to take blowers out of service to reduce oxygen input.

(5) Toxicity in the mixed liquor can prevent the micro-organisms from using the dissolved oxygen present, which also produces high effluent dissolved oxygen. This condition is temporary: it is followed by a rapid fall in dissolved oxygen and deteriorating effluent quality. The source of the toxic substance must be identified and eliminated.

3. Sludge Floating on the Clarifier Surface

Sludge flotation means that sludge inside the clarifier becomes acidified or denitrifies and floats to the surface. The floating sludge itself is not poor quality: its biological activity and settling performance are normal. The cause is that this otherwise healthy sludge stays too long in the clarifier, where dissolved oxygen is gradually consumed and acidification follows, producing gases such as hydrogen sulphide that attach to the flocs, lower their density and make them float. Where the sludge age is long and nitrification has taken place, the mixed liquor entering the clarifier contains a large amount of nitrate; because there is not enough dissolved oxygen in the clarifier, the nitrate is reduced to nitrogen gas, which again lifts sludge to the surface.

Floating sludge is controlled in two ways. First, waste sludge should be removed promptly and the return sludge flow increased, so that sludge does not stay too long in the clarifier. Second, aeration at the end of the aeration tank should be strengthened so that the mixed liquor entering the clarifier carries more dissolved oxygen and the sludge does not become anaerobic or anoxic. Where the cause is denitrification, the waste sludge flow can be increased further to reduce the sludge age, controlling nitrification and therefore denitrification.

4. Black Sludge Rising in Blocks

Black sludge rising as blocks on the clarifier surface is usually the result of sludge putrefaction. Insufficient aeration leaves the sludge anoxic in the clarifier, or a large sludge production combined with low wasting extends the retention time; an unsuitable hopper shape prevents sludge from sliding down; blocked suction pipes or dead zones the collector cannot reach all leave sludge deposited for long periods in part of the tank. Anaerobic metabolism then produces large quantities of hydrogen sulphide and methane, which envelop the sludge and make it rise in large black masses. Putrefactive flotation differs from ordinary flotation: the sludge turns black, rots and gives off a strong odour.

The remedy is to ensure that waste sludge is removed promptly, to eliminate faults in the sludge removal equipment, to clear sludge from the tank walls and dead zones, to reduce the degree of nitrification in the aerobic system, to increase the return sludge flow, and to prevent putrid sludge from other treatment units entering the clarifier.