MANAGING DISCHARGE COMPLIANCE WITH “GREENER” CHEMISTRY
Discharged residues of coagulants and flocculants used for the treatment of effluent streams and stormwater run-off are under increasing scrutiny for potential adverse impacts on downstream flora and fauna. Synthetic agents pose particular problems because they cannot readily be monitored in the field, so that non-compliant discharges may go undetected until laboratory testing furnishes results perhaps a week later. Our experiences with deployment of naturally derived biopolymers, often in unique combinations with one another or with inorganic coagulants, make a compelling case for the advantages of these formulations. Not only do they offer superior overall system cost-effectiveness, but they rapidly and harmlessly degrade in the environment; furthermore, discharge residues can be readily monitored on-site to be sure of ongoing compliance with regulatory needs. An example of the benefits of one such formulation will be illustrated, enabling operators to cope with rain events that were seriously exacerbating effluent management at an aggregate mining site in BC. Site management through serial retention ponds was frequently overwhelmed, allowing discharge levels to reach up to 20,000 NTU. Sequential feed pump injection of two proprietary formulations, both containing biopolymers (the first synergistically enhancing a conventional inorganic flocculent) consistently enabled discharge, after sand bed filtration, in the region of 10 NTU (requirement for release is 50). Capable of dealing with peak flows of 8000 gpm, the system can now cope with 1 million g, and accumulate 30,000 g of mud, routinely removed by an onsite mud-handling operation. On-site monitoring for the discharge of the major biopolymer, Chitosan, permits detection down to 100 ppb. Laboratory bench modeling of sand-bed filtration of floccules created with Chitosan-based formulation has shown preferential retention of these in the uppermost portion of the filtration medium. This characteristic allows for more efficient management of backwash requirements and faster flow rates. Data on site-testing sensitivity and floccule retention will be illustrated, as evidence in support of the growing case for biopolymer use in environmentally responsible industrial water treatment.