On-site Wastewater Systems - Risks and Insights into Their Function
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Bronwyn Humphries (ESR), Gemma Langley (ESR), Andrew Pearson (ESR), Marta Scott (Environment Canterbury) and Louise Weaver (ESR)
On-site wastewater management systems (OWMS) release chemical and microbial contaminants into the environment, potentially posing risks to surface water, groundwater, and human health. Although OWMS effluent quality has been characterised internationally, in Aotearoa (New Zealand), the chemical and microbial quality of effluent is poorly characterised. Thus, water regulators and engineers for OWMS manufacturers typically rely on international data to inform policy and estimate risks to receiving environments and human health. However, international data is unrepresentative of the OWMS designs typically present in Aotearoa and therefore may differ in treatment capacities and discharge quality.
To address this knowledge gap, we present effluent quality data from three sources sampled within Waitaha/Canterbury:
• Long-term field-scale OWMS research site in North Canterbury
• Composite samples (taken across 4-5 days) from a primary and secondary treatment OWMS
• 30 discrete samples from a mix of primary and secondary treatment OWMS across Canterbury
Results from the 30 OWMS discrete sample locations include Escherichia. coli (E. coli) concentrations that ranged between 5.4 x 103 and 2.4 x107 MPN/100mL for primary treatment systems and between 2 x102 and 9.8 x106 MPN/100mL for secondary treatment systems. The mean E. coli results for primary and secondary treatment systems were 2.9 x106 and 8 x105 , respectively. Total phosphorous concentrations ranged from 7 – 91 mg/L and 2 – 20 mg/L for primary and secondary treatment systems, respectively. Mean total phosphorous concentrations for primary and secondary treatment systems were 17 and 12mg/L, respectively. Total Kjeldahl Nitrogen (TKN) concentrations ranged from 35– 300 mg/L and 2 – 98 mg/L for primary and secondary treatment systems, respectively. The mean TKN concentrations for primary and secondary treatment systems were 98 and 23 mg/L, respectively.
This OWMS effluent quality data contributes to the understanding of the composition of effluent entering the receiving environment. The composition of this discharge from OWMS in Aotearoa is relatively unresearched in a New Zealand context, therefore, this research provides critical information to decision-makers concerned with the impact of OWMS on water quality. Additional insights can be achieved by coupling chemical and microbial loading rates with OWMS location information to estimate the significance of the contaminant contribution to the environment to direct policy, estimate impacts on catchment and regional scales, and inform wastewater infrastructure decisions.