Aluminium speciation: implications of wastewater effluent dosing on river water quality

摘要

Aluminium-based coagulants can be used during wastewater treatment to reduce phosphate loads discharged to surface waters. Aluminium is recognised as potentially toxic to fish under pH conditions where precipitation at the gill surface may occur. This is reflected in the way in which the proposed aluminium Environmental Quality Standard (EQS) has been set as a limiting concentration of 10 mu g/L for waters of a pH value below 6.5 and a limit of 25 mu g/L for waters of pH greater than 6.5. Field and laboratory studies have been undertaken to examine aluminium speciation in wastewater treatment works effluent and receiving waters under a variety of pH conditions. The practicalities of implementing the proposed EQS values were assessed. Field observations indicated consistent compliance with the proposed EQS aluminium levels for waters of relatively high pH value. Laboratory studies showed that for waters of pH value significantly less than 6.0, reactive aluminium concentrations naturally present through dissolution of aluminosilicate minerals tended to exceed levels discharged in a sewage effluent. After mixing between effluent and receiving water, changes in aluminium speciation were largely complete within a one hour period.