Wastewater is typically considered something to quickly dispose of, but in fact, it is an ‘environmental trifecta’ of recycled water, organic biosolids, and methane gas.
By Robert C. Brears
San Antonio Maximizing its Resources
To reduce San Antonio’s dependency on the drought-sensitive Edwards Aquifer for the city’s water needs, the San Antonio Water System (SAWS) has developed the largest direct recycled water system in the United States. More than 130 miles of pipeline deliver up to 35,000 acre-feet per year of high-quality recycled water for non-potable uses including golf courses, parks, and commercial and industrial customers throughout the city. The system is also designed to supplement flows in the San Antonio River and Salado Creek. In addition, recycled water supplies up to 50,000 acre-feet per year for CPS Energy’s power plants.
The SAWS Composting Program uses a combination of wood chips and biosolids to form compost, with the program diverting up to 150,000 cubic yards of wood chips from landfills each year while the biosolids component, the by-product of water recycling, diverts up to 208,000 cubic yards of biosolids from landfills each year. SAWS has partnered with New Earth to conduct all compost-related activities. This provides better service for compost customers and lowers the costs for SAWS ratepayers.
SAWS is harnessing methane gas generated during the wastewater treatment process as a renewable energy source. The biogas – which is 60% methane – is a by-product of the anaerobic digestion process from biosolids, with San Antonio producing around 140,000 tons of biosolids per annum. SAWS has partnered with Ameresco, Inc., a national energy company that focuses on renewable energy. Since 2010, Ameresco has processed more than 1.5 million standard cubic feet of biogas a day and delivered a minimum of 900,000 cubic feet of natural gas each day to the nearby commercial pipeline to sell on the open market. Economically, SAWS receives around USD 200,000 in annual royalties from the sale of the biogas, reducing the costs of SAWS operations and keeping rates affordable, while environmentally the harnessing of biogas instead of flaring reduces 19,739 tons of carbon dioxide each year.
Helsinki’s Circular Wastewater Treatment Plant
Helsinki’s Viikinmäki Wastewater Treatment Plant processes wastewater from industry and 800,000 people in Helsinki and its 7 neighboring municipalities. The average wastewater flow is 280,000 m3/day and peak flow is 700,000 m3/day. Of the waste, around 85% is domestic wastewater and the rest is from the industry. The sludge generated in the treatment process is processed in digestion tanks, with the methane gas generated in the digestion process utilized in energy generation to produce electricity and heat. The electricity generated corresponds to 70% of the wastewater treatment plant’s electricity needs, with the aim of reaching 80% soon.
Meanwhile, the digested and dried sludge from the wastewater treatment plant is transported to the Metsäpirtti composting field in Sipoo. The sludge is mixed with peat in a ratio of 1:1. The mixture is composted in stacks for around 6 months, after which sand and biotite are added to the mixture. In the last stage of the process, the soil mixture is screened (with the screen size set at 20 mm). Following which, Metsäpirtti soil products for gardens is created, enabling the wastewater treatment plant to recycle around 580 tons of phosphorus and 620 tons of nitrogen per annum.
Stockholm’s Wastewater Powering Vehicles
In Stockholm, the two sewage treatment plants, Henriksdal and Bromma, serve more than one million people and industries in the city plus surrounding municipalities. During the sewage treatment process, the organic material is separated in the form of sludge from the water. In total, the two plants produce around a million tons of sludge per year.
When the sludge is digested biogas is formed, providing a steady stream of vehicle fuel: currently, around 17 million cubic meters of crude gas is produced which is sold to Scandinavian Biogas, who then transform the raw gas into vehicle gas. The gas that is not converted to vehicle gas is used for heating and electricity generation. Most of the gas produced at Henriksdal is used by SL’s inner-city buses. Meanwhile, vehicle gas from Bromma is sold, partly from a tank outside the plant and partly at other filling stations in the city, to taxis, private cars, buses, and waste trucks. Overall, the biogas mitigates more than 22,000 tons of carbon dioxide emissions annually.
The sludge has a high phosphorous content and after dewatering around 80,000 tons of sludge per year remains which can be used as fertilizer. The goal is to return at least 70% of the sludge to agriculture, which corresponds to 120 tons of phosphorous per annum. The other 30% is used to restore land areas around mines. The sludge is certified according to Revaq, which is a quality certification scheme that assures that plant nutrients from wastewater is produced responsibly and does not contain harmful substances.
Stockholm’s purified wastewater has a temperature of 7-20 degrees Celsius. Before it is released in Saltsjön, heat from the water is recycled as district heating at Hammarby and Solna Heating plants. This process reduces the heat in the water down to 0.5-4 degrees Celsius.
Berlin Maximizing the Value of Wastewater
Berliner Wasserbetriebe has committed itself to achieve climate neutrality by 2050. Contributing to this goal, the utility’s Schönerlinde sewage treatment plant is turning sewage sludge into sewage gas to generate power and heat. Additionally, the utility has constructed three wind turbines, with a capacity of 2 MW each, as well as two micro gas turbines to complement the plant’s CHP unit. Overall, around 84% of the energy required by the plant is produced internally, saving up to 13,000 tons of carbon emissions per year.
Berliner Wasserbetriebe has developed a patented process for recovering phosphorus from its sewage treatment plants. The recovered phosphorus is sold under the brand name ‘Berliner Pflanze’ (Berlin Plant) to horticulture and agriculture producers in the surrounding areas of the city. Several years ago, Berliner Pflanze won the GreenTec Award for environmentally friendly recycling products.
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