Reuse of lagoon effluents in agriculture by post-treatment in a step feed dual treatment process

The main constraint in lagoon treatment is the high suspended solids (SS) in the effluents, which is primarily due to high concentrations of algal cells in the finished effluent. The objective of this study was to remove turbidity originating from algae present in oxidation pond effluents by an easy and inexpensive method. For this reason, a novel lab-scale step feed dual treatment (SFDT) process was developed and the effectiveness of the trickling filter (TF) unit within the system in removing algae and organic matter was investigated. The SFDT process developed in this study is a unique and inexpensive way to scavenge algae from oxidation pond effluents. As opposed to earlier and somewhat unsuccessful studies where pond effluent post treatment was tested on oncethrough trickling filters, in this study pond effluents were directed to a step fed TF, so as to provide a dual treatment. Step feeding provided the necessary substrate to maintain a biofilm in TF, thereby affecting organic particles interception. The stabilization pond was not simulated in the study since the main focus was on the behaviour of the TF unit. The hydraulic loading rate (HLR) (0.5–2–4 m3/m2 day), influent COD (150–550 mg/L) and influent Chl-a concentrations (250–600 µg/L) were selected as operational variables. It was observed that, in general, removal percentages for turbidity, Chl-a, SS and COD increased considerably with the decreasing HLR, such as the removal efficiency of Chl-a was increased from 89.4% to about 97% when HLR was decreased from 4 m3/m2 day to 2 m3/m2 day. As a result, trickling filter produced clear effluents, with less than 2 NTU and the removal efficiency of turbidity being higher than 88%, and also removal percentages for Chl-a were higher than 95% for most of the cases.

In this study, the removal of Chl-a, SS and turbidity in a lab-scale TF unit of a SFDT process was investigated. The SFDT process developed in this study is a unique and inexpensive way to scavenge algae from pond effluents. Following are the conclusions drawn from this study: The SFDT process produced clear effluents with <2 NTU, for most of the experimental sets tested. WHO agricultural reuse standard calls for less than (or equal to) 2 NTU in the effluents which are to be used for unrestricted irrigation. Most important parameter affecting the performance of TF in SFDT was found to be HLR. Highest removals were obtained at the low HLRs. Data obtained from the experiments showed that when HLR (m3/m2 day) was increased from 0.5 to 2, a slight decrease was observed in Chl-a, NTU and COD removals; though more than 90% removal of each was attained in every case, except for the COD. The lowest removal efficiencies were observed for all the quality parameters when hydraulic loading rate was 4 m3/m2 day. The SFDT system should require minimum of maintenance, and not more than oxidation ponds and trickling filters together. Both processes are known to be the least maintenance requiring amongst the alternative waste treatment processes.