Wastewater Treatment Efficiency by a Freshwater Phylactolaemate Bryozoan and Experimental Feeding with Protozoa

The wastewater treatment ponds of the King’s Royally Initiated Laem Phak Bia Environmental Research and Development (LERD) Project in west-central Thailand provide habitats for freshwater bryozoans, which are colonial invertebrate animals. Bryozoans sieve food particles out of the water using a retractable lophophore and can play an important ecological role in wastewater treatment. In this unique environment, we: (1) investigated the efficiency of a phylactolaemate bryozoan (Plumatella casmiana) in wastewater treatment, measured by BOD5, chlorophyll a and turbidity; and (2) determined the role of protozoans in the diet of the bryozoan P. casmiana. Comparison of growth rate and fecal pellet characteristics between protozoan-fed bryozoans and phytoplankton-fed bryozoans was investigated. At the end of our wastewater treatment experiment, water quality parameters were markedly improved in the treatment with bryozoans compared to the control (without bryozoans). The treatment efficiency levels for BOD5, turbidity, and chlorophyll a were 24.04%, 59.21%, and 55.13%, respectively. The growth rates of bryozoans in the experimental treatment increased over time. Our study also revealed that this bryozoan can feed on a diet of protozoans under experimental conditions. However, the average daily growth rate of protozoan-fed bryozoans -20 zooids per day was lower than that of phytoplankton-fed bryozoans 19 zooids per day. This may have been due to incomplete digestion of protozoans or insufficient nutrition in the bryozoans. The results from this study provide better understanding of bryozoan ecology and their role in wastewater treatment systems.