Solar PV adoption in wastewater treatment plants: A review of practice in California

This is the first study to assess the current status of solar photovoltaic (PV) adoption across a range of wastewater treatment plant sizes, and to identify the opportunities for solar PV in the wastewater sector. It quantifies solar PV contributions to the energy demand of the wastewater treatment plants and improves knowledge of sector-specific factors influencing PV uptake. California was used as a case study due to its high commitment to solar power and the high data availability. The study compiled and examined data on multiple wastewater treatment plant attributes from 105 Californian plants, representing 78% of total state flows. The analysis focused on the effect of three sector-specific influencing factors: size of wastewater treatment plant, presence/absence of anaerobic digestion and geographical location (urban vs rural). Solar PV adoption was observed to vary significantly with the size of the wastewater treatment plants. Of the 105 plants analysed, 41 installed a solar PV system. Of these 41, 39 were installed in wastewater treatment plants with a flow rate below 50 mega gallons day−1 (MGD). Only two plants with flow above 50 MGD had solar PV installed. In wastewater treatment plants with a flow rate above 5 MGD, solar PV was primarily installed in hybrid configurations with anaerobic digestion. In these plants, biogas contributed 25–65% to the overall energy demand, while solar provided 8–30%. In wastewater treatment plants with a flow rates below 5 MGD, solar PV often represented the only source of renewable energy, producing 30–100% of the energy demand of these plants. Across all the plants analysed, 1 MW was the most adopted solar installation size and solar PV installations were mostly found in wastewater treatment plants in rural settings. While acknowledging multiple other factors of potential influence, these results demonstrate the role of solar PV in wastewater treatment plants under three sector-specific influencing factors. The results will support the sector in making informed decisions over solar PV investments, helping wastewater utilities to transition towards sustainable management practices.