A study on the interdependency of hydroponic wastewater quality and the energy and costs for onsite treatment

Abstract

Hydroponic growing methods have the potential to use less water while producing higher yields when compared to traditional soil-based agriculture. However, hydroponic wastewater is a nutrient dense effluent that can be harmful to the environment if not managed properly. Onsite treatment and reuse of hydroponic wastewater would avoid thousands of gallons a month of freshwater use while preventing this effluent from harming receiving streams. The efficacy, energy requirements and costs of some treatment methods are not well known. To assess the efficacy of select treatment methods, the contaminants in hydroponic wastewater were measured using samples of hydroponic wastewater collected from a greenhouse test facility and analyzed by an environmental laboratory. Contaminants evaluated were Total Dissolved Solids (TDS), calcium, potassium, magnesium, total phosphorus, nitrogen, and Total Organic Carbon (TOC). Wastewater samples were also treated onsite using a sand filter, a granular activated carbon (GAC) filter, and reverse osmosis (RO). Samples of wastewater treated by these methods were then re-analyzed by the environmental laboratory and post-treatment concentrations of the studied contaminants were recorded. Sand and GAC filtration were shown to be essentially ineffective for contaminant removal due to the high concentrations of metals in the hydroponic wastewater. Reverse osmosis was the most effective treatment method, removing an average of 85% TDS concentration in wastewater samples. The results from the water quality analysis showed that Reverse Osmosis was the only treatment method that effectively removed the large concentration of metal contaminants in the wastewater. For this reason, only reverse osmosis was analyzed for energy and cost requirements for onsite treatment of hydroponic wastewater. The energy requirements to treat hydroponic wastewater onsite by reverse osmosis ranged from 3 to 43 kWh per day, depending on the facility size and percent of water treated. The annualized cost of treatment ranged from $0.63 to $2.83 per thousand gallons of water treated. Finally, a cost savings from reduced water bills analysis was also performed using local water utility prices in Austin TX. Based on the assumptions made for water meter size and monthly water use, it was found that financial savings could be achieved in all facility sizes with a payback period of 7 to 24 months if facilities utilized municipal water as their water source. These results can help to determine whether recycling hydroponic wastewater is feasible within financial and energy constraints as a way to avoid discharging harmful effluent and using thousands of gallons of source water each month.