ORGANIC-MATTER, NUTRIENTS, AND OPTICAL CHARACTERISTICS OF SEWAGE LAGOON EFFLUENTS

The effluent characteristics of 11 domestic sewage waste stabilisation lagoons designed to national specifications were examined, with emphasis on their optical properties, biochemically active constituents, and receiving water impacts. Measurements of 26 variables were analysed and compared with our previous (1989) survey of lagoon effluent quality in New Zealand. There was considerable variation in effluent composition of the lagoons, although seasonality was not marked (< 3-fold change between summer and winter). The filterable proportion of total BOD5 appeared to be a useful indicator of lagoon performance. Nutrient levels in lagoon waters were high (overall medians were 5.1 g m(-3) for dissolved reactive phosphorus and 10.5 g m(-3) for dissolved inorganic nitrogen). The concentration of ammoniacal-N, which comprised most of the dissolved inorganic nitrogen, was highly variable (ratio of 95 to 5 percentile = 1900); this is of concern as a toxicant both to algae within lagoons and to aquatic life in receiving streams. Aquatic humus and algal biomass both contribute to the restricted light penetration (median euphotic depth 0.35 m) into lagoon waters (typically 1.2 m depth), apparently resulting in frequent light limitation of algal photosynthesis. Pond effluent character seems to be influenced strongly by sunlight and wind, via their effects on lagoon solids (largely algal biomass and associated detritus). Dilution of lagoon effluents by 40-field would ensure that receiving streams meet existing guidelines for most attributes of concern, but > 100-fold dilution may be required to prevent visual clarity impacts, and several hundred-fold dilution to prevent benthic algal proliferations.