Chemical and Microbiological Quality of Effluents from Different On-Site Wastewater Treatment Systems across Finland and Sweden

Treatment efficiency of package plants for on-site wastewater treatment in cold climates

Package plants (PP) are implemented around the world to provide on-site sanitation in areas not connected to a sewage network. The efficiency of PP has not been comprehensively studied at full scale, and the limited number of available studies have shown that their performance varies greatly. Their performance under cold climate conditions and the occurrence of micropollutants in PP effluents have not been sufficiently explored. PP are exposed to environmental factors such as low temperature, especially in cold regions with low winter temperatures and deep frost penetration, that can adversely influence the biochemical processes. The aim of this study was to investigate the treatment efficiency and possible effects of cold temperatures on PP performance, with focus on traditional contaminants (organics, solids, nutrients and indicator bacteria) and an additional assessment of micropollutants on two PP. Eleven PP hosting different treatment processes were monitored. Removal of biological oxygen demand (BOD) was high in all plants (>91%). Six out of the 11 PP provided good phosphorus removal (>71%). Small degrees of nitrification were observed in almost all the facilities, despite the low temperatures, while denitrification was only observed in two plants which achieved the highest nitrification rates (>51%) and had sludge recirculation. No strong correlation between wastewater temperature and BOD, nutrients and indicator bacteria concentration in the effluents was found. The high data variability and the effects of other process parameters as well as snow-melt water infiltration are suggested as possible reasons for the lack of correlation. However, weak negative relations between effluent concentrations and wastewater temperatures were detected in specific plants, indicating that temperature does have effects. When managed adequately, package plants can provide high BOD and phosphorus removal, but nitrogen and bacteria removal remain challenging, especially at low temperatures. Pharmaceutical compounds were detected in the effluents at concentrations within or above ranges reported for large treatment plants while phthalate ester concentrations were below commonly reported effluent concentrations.

Efficiency of Private Household Sand Filters in Removing Nutrients and Microbes from Wastewater in Finland

Sand filters have been shown to be an economic and effective solution for wastewater treatment in private households, although the removal of phosphorus (P) may be insufficient. However, P removal can be improved by adding a P-adsorbing material, such as biotite, into the sand filters. The physico-chemical characteristics and the microbial quality of the effluents of family-scale sand filters without adsorbing media (SF), sand filters with a biotite layer (B), and sand filters with a modular filter (MB) were followed for one year. Sand filters with a biotite layer displayed the highest capacity to remove nitrogen (N) and biological oxygen demand BOD7. The efficiency of these filters did not depend on the age of the filter or the season. The P load of the effluent did not differ between sand filters with and without a biotite layer, but the modular sand filter failed to adequately reduce P. The treatment efficiency of sand filters without biotite decreased with increasing age. These private household sand filters can be a good way to treat domestic wastewaters, since these generally comply with the minimum requirements of a Finnish Decree (157/2017). However, enteric viruses (noro- and adenoviruses) were commonly found in the effluents, and the numbers of Escherichia coli were often above 103 colony forming units (CFU) 100 mL−1 (limit for the the EU Bathing Water Directive 2006/7/EC) for good water quality), signifying a risk of microbial contamination of nearby drinking water wells, as well as bathing and irrigation waters.