Abstract
Cationic polymers (e.g. polyacrylamide) having a high molecular weight require a much lower dose than inorganic metal salts (e.g. AlCl 3 , FeCl 3 , or Al 2 (SO 4 ) 3 ) to produce dense compact flocs with good settling characteristics. This study investigated the effect of a continuous low dosing of cationic polyacrylamide (PAM) to achieve efficient algal removal from a 1‐ha High Rate Algal Pond (HRAP) treating domestic wastewater in New Zealand. Firstly, standard jar tests were conducted in laboratory conditions. A wide range of PAM dose rates (0.25–9 mg/L), mixing periods (60–250 s) and settling times (0–120 min) were tested on HRAP effluent samples to determine optimal operational parameters for the 1‐ha HRAP in terms of TSS and turbidity removal. PAM addition effectively promoted flocculation of the wastewater algal biomass, removing 78% of TSS (from 135 to 30 mg/L) and 70% of turbidity (from 41 to 12 NTU) at a dose of only 4 mg/L. The jar tests determined that ∼120 s of vigorous mixing followed by 15‐min of settling were sufficient to promote the formation of dense/settleable algal flocs. Continuous addition of PAM to the hectare‐scale HRAP effluent at the 4 mg/L laboratory determined dose rate was found to improve TSS removal in the Algal Harvest Pond (AHP) by ∼55% (from 13% to 68%). The TSS concentration and turbidity of the AHP effluent were consistently maintained E. coli removal (from 1 × 10 5 to 1 × 10 4 MPN/100 ml after settling) was also achieved in the AHP as a result of the algal floc formation and removal. Overall, this study demonstrated that addition of low levels of the chemical flocculant (e.g. cationic polyacrylamide) to the HRAP effluent can greatly improve algal solid removal (at least 50%) with a minimum chemical cost (∼$0.05 NZD/m 3 of wastewater).