Reproduction of Staurastrum sp. within a water treatment plant caused by the recycle of combined sludge water and backwash water: a field investigation

It is well known that the recycle of sludge water and filter backwash water in drinking water treatment plants (WTP) may cause water quality problems including suspended solids, pathogenic protozoans, dissolved organic matters, and disinfection by-products. However, it is still uncertain whether the recycle process can cause algal problems in a WTP with algal-laden raw water. A field survey was performed in a WTP, located in Tianjin, China, to investigate the variation of algal species and density during the treatment and recycle process. The results showed Staurastrum sp., a non-dominant species in raw water, regrew and became the unique dominant species in the combined sludge tank (CST). The reproduction of Staurastrum sp. increased the treatment loading, clogged the filter, and deteriorated the quality of finished water. Bench-scale experiments indicated chlorination was inefficient to inactive Staurastrum sp., which resulted in that living cells of this alga can enter into the CST. Culture experiments indicated Staurastrum sp. favors a moderate concentration of Fe (0.6–2.0 mg/L) and grows slowly under a low concentration (0.1 mg/L). Thus, the high concentration of Fe (0.6 mg/L) originated from the residual coagulant facilitated the reproduction of this alga in the CST. Cutting off the recycle pathway and destroying the favorable growing conditions in the CST may be feasible methods to control the reproduction of Staurastrum sp. in the WTP. This study showed the recycle of sludge water and filter backwash water can cause algal problems in WTPs, which should be an important issue for drinking water treatment researchers and engineers.