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Wastewater resources can be valuable if reclaimed properly

Anaerobic digestion produces valuable biogas, which can be converted into renewable energy. Photo: ADI Systems.

By Shannon Grant

Operators of wastewater treatment plants can combat both wasted natural resources and unnecessary expenses, by shifting their focus towards resource recovery. Green energy, clean water, and nutrients can all be harnessed from wastewater.

This treatment plant saves hundreds of thousands of dollars with biogas utilization. Photo: ADI Systems.
This treatment plant saves hundreds of thousands of dollars with biogas utilization. Photo: ADI Systems.

Tapping into the power of biogas

Anaerobic digestion produces valuable biogas, creating an easy opportunity to convert waste into energy. On-site energy production is one of the most practical ways plants can become more sustainable. Biogas can be used in boilers to produce heat, or burned in cogeneration engines to produce both electricity and heat. In some cases, it can be sold to the energy network for utility credits. These smart strategies also help displace fossil fuel use.

The first step to harnessing the power of biogas is to capture it with a gas-tight tank or lagoon cover. Exactly how much energy can be captured depends on the quantity of wastewater being treated and its strength. But typically, an anaerobic digestion system can capture thousands of cubic metres of biogas per day.

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In many cases, biogas contains hydrogen sulfide (H2S), which must be removed in order to reduce health risks, equipment corrosion and harmful sulfur emissions. This can be achieved using either biological or chemical scrubber technologies.

Once treated, biogas captured from anaerobic digestion can be used in many applications, including: internal combustion engines, gen-sets, microturbines, fuel cells, sludge dryers, and boiler and steam generating systems.

Biogas produced from the anaerobic wastewater treatment system at one WWTP is captured and burned in two 1,000 kW gas engines to generate electricity. The electricity helps power the wastewater treatment process, saving hundreds of thousands of dollars in annual electricity costs. It also generates revenue by selling excess electricity to the local utility.

Nutrient recovery

Nutrient-rich waste anaerobic sludge (WANS) from the anaerobic digestion of wastewater can be recycled as fertilizer. The production of WANS for use or sale can also contribute to the economics of a biogas system.

Making better use of biosolids places essential nutrients and water back into the soil. The high levels of macro– and micro-nutrients in WANS presents an environmentally-sound alternative to mineral commercial fertilizers. Nutrients in WANS, including nitrogen and phosphorus, are plant-accessible, increasing nutrient absorption and reducing pollution by the non-utilized portion of the nutrients. When applied on land, these nutrients re-enter the food chain via uptake by plants and crops, creating a closed-loop nutrient cycle. Land application also reduces hauling costs and the associated vehicle emissions.

Reduce, reuse, recycle water

The right on-site treatment system can transform treated wastewater into a reliable alternative water source, helping address water shortages, while reducing operational costs. Water conservation further minimizes the demands on freshwater sources and demonstrates environmental stewardship.

Aerobic technologies such as membrane bioreactors are a good starting point for water reuse projects because they can produce a very high-quality effluent. The treatment process results in extremely low nitrogen, phosphorus, biochemical oxygen demand, and total suspended solids concentrations. The physical barrier in membrane bioreactors ensures complete solids retention and process stability. With the right technology, potable water reuse applications are also an option, although this usually requires reverse osmosis.

Treated wastewater can be used in a number of applications, including: makeup water for cooling towers and boilers, equipment cleaning, vehicle washing, irrigation, air conditioning, toilet flushing and fire protection.
Noosa’s Yoghurt has an on-site wastewater treatment system. Its aerobic membrane bioreactor allows the dairy processor to comply with strict environmental regulations, and generate an effluent suitable for reuse on-site or direct discharge. Treated water is reused for cleaning cow barns and irrigating crops.

Wastewater treatment plants that have made the shift to treating wastewater as a resource rather than treating it like waste are minimizing costs and realizing the first-hand benefits of rethinking their perspective on wastewater. Water reuse, waste reduction, and energy generating initiatives are helping protect both the environment, and a plant’s bottom line.

Shannon Grant is with ADI Systems. This article appears in ES&E Magazine’s October 2017 issue.

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