Even the most well-run wastewater treatment plants may experience effluent quality issues in the spring, when reddish blooms often appear in the final clarifiers and cause higher TSS. These reddish blooms are caused by a proliferation of ceriodaphnia (water fleas). This is the same organism used in the effluent toxicity testing required by discharge permits. The ceriodaphnia thrive in the clear water of final clarifiers, feeding on algae growing on the sunlit submerged surfaces. As the mixed liquor temperatures warm up in spring, a healthy ceriodaphnia population proliferates as temperatures reach 18°C. The water fleas can interfere with effluent TSS testing, causing high results. For plants with effluent filtration, the bloom of water fleas can greatly shorten filter runs, resulting in more frequent backwash cycles and higher hydraulic recycling.
The key to controlling waterflea blooms is keeping final clarifiers clean of algal growth to deprive the waterfleas of their food source. Chlorinated sprays and brushing of effluent troughs will reduce algal growths. Clarifiers with dedicated trough cleaning systems (brushes or sprays) will generally not experience large blooms of water fleas. Plants with multiple final clarifiers can take units out of service and lower the water level below the effluent troughs to allow the algae-covered surfaces to dry out, killing the algae prior to returning to service. The ultimate control of algae is provided by clarifier covers that prevent sunlight from reaching the clarifier surfaces, since the dark conditions are not conducive to growths of either algae or water fleas.