Role of freshwater bryozoans in wastewater treatment ponds at Laem Phak Bia Environmental Research and Development Project site, Phetchaburi province, Thailand

Abstract

Roles of freshwater bryozoans in wastewater treatment ponds have been recognized. However, there are still limited studies on this issue. Established in 1991, the Laem Phak Bia Environmental Research and Development Project (LERD Project) in western central Thailand has provided natural oxidation of urban wastewater via treatment ponds. These oxidation ponds are suitable habitats for freshwater bryozoans. However, research on bryozoans in these wastewater treatment ponds has received little attention. Thus, this research: 1) studied the biodiversity of the bryozoans in five wastewater treatment ponds, 2) determined the impact of wastewater on the growth rates and survival of the bryozoans and 3) investigated the role of bryozoans as filter feeders. Field surveys and laboratory experiments were conducted in 2018–2019. The results identified three species of freshwater bryozoans (Plumatella casmiana, P. vorstmani, P. vaihiriae) in the ponds. The water quality (biological oxygen demand) in the oxidation pond affected the survival and growth rates of the bryozoans. Ponds 1 and 2 had high organic loadings leading to the death of most bryozoans. In contrast, ponds 3–5 supported colonies of bryozoans. The different water quality conditions reflected the different growth rates of the bryozoans. Furthermore, depth was not the key factor that impacted the growth rates of the bryozoans since there was no significant difference in the growth rates of the bryozoans between the shallow (exposure to light) and deep zones (no light). Lastly, the filtration rates of the bryozoans were notable, with P. casmiana and P. vorstmani removing up to 74% of the suspended particles.