Improved fecal coliform decay in integrated duckweed and algal ponds

Abstract

Algal growth in wastewater ponds has two contradicting effects on fecal coliforms (FC) decay. On the one hand, algal photosynthesis increases the FC decay due to increased pH and DO. However, on the other hand, attenuation of solar radiation by algal matter reduces the decay rate. It was therefore investigated if suppressing algal development could enhance the FC removal efficiency. Limiting the algal growth was accomplished by inserting duckweed ponds in between a series of algal ponds. Duckweed ponds are modified stabilization ponds, covered with a mat of small floating plants, that are known to remove algal from algal pond effluent. The FC decay in a series of five shallow algal ponds was compared to FC decay in an integrated system of algal and duckweed ponds. The integrated system consisted of five mini-ponds (30 cm depth) in series: duckweed pond – algal pond – duckweed pond – algal pond – duckweed pond. The environmental factors that are known to affect FC decay were monitored and related to FC decay rates. In the algal ponds of the conventional system the light attenuation by algal matter became rate-limiting for the FC decay. In the integrated system, the algal concentration in the algal ponds was reduced by the intermediary duckweed ponds. This was shown to increase the FC decay in the algal ponds of the integrated system considerably, compared to the FC decay in the algal ponds of the conventional system. An improved system of duckweed and algal ponds is proposed, that is expected to reduce significantly the area requirements of pond systems.