Although the concept of energy-neutral wastewater treatment facilities is relatively new, there are several examples of plants that have adopted strategies to approach that goal or are taking the required steps by investing in the technologies needed to reach it.

Net energy neutrality or electrical grid self-sufficiency (also called electricity autonomy) is often a key component of modern plant design, and it is one that progressive industry suppliers consider a template for the plant of the future.

A number of wastewater treatment plants around the world are making substantial progress toward net energy neutrality, and their experiences provide lessons for facilities in North America.

Key requirements

In assessing the potential to convert an existing plant to net energy neutrality, there are a few key requisites and several other main considerations. The requisites may include primary sedimentation, anaerobic digestion, a way to collect and use biogas, and sufficient volatile solids content in biosolids stream to be digested.

A key question is whether there is a way to improve biogas production from the existing biosolids, or provide a supplemental energy source for co-digestion, such as FOG or food waste. These are all key elements in a plan to maximize biogas production.

The net increase in production and use of the biogas, in turn, can significantly offset the net electrical consumption. Aside from the need for additional unit operations to facilitate the use of biogas, the wastewater and biosolids characteristics have a large influence on whether a plant can approach energy neutrality.

Since the oxidation of organics and ammonia is necessary, aeration is required and represents a large share of the electrical consumption. Advances in high-speed turbo blowers have improved wire-to-air energy consumption efficiencies and, when combined with an efficient diffuser system, have enabled improvements in overall oxygen transfer efficiencies (kW/kg O2).

Meeting the permit

Another key consideration, often overlooked in the planning phases, is the effluent quality requirement. In general, electrical self-sufficiency is easily achievable where permits require only TSS and BOD removal. A requirement to remove nitrogen can reduce the ability to achieve full electricity autonomy by as much as 40 percent.

Improvements on this are possible if biogas production can be enhanced, such as by supplementing digester feed with organic waste. Net energy balance can be further enhanced with improvements to primary clarification to achieve higher TSS removal, and with reductions in aeration system energy consumption (using higher-efficiency blowers).

Several plants are already supplementing digester feed with FOG or food wastes to increase biogas production. Alternatively, pretreatment of sludge via thermal hydrolysis or enzyme hydrolysis has proven effective in boosting gas production.

Relatively new technologies for treatment of digested sludge dewatering sidestreams, such as those based on partial nitration and deammonification via the Anammox bacteria pathway, are becoming more common and will further reduce energy consumption related to the oxidation of NH4 by as much as 10 percent.

Cases in point

Here are a few examples of treatment plants around the world that are approaching energy neutrality.

Marquette-lez-Lille, France

The Marquette wastewater treatment plant, serving a population equivalent of 620,000, is being expanded with the goal of maximizing electricity self-sufficiency while reducing biosolids volume. The new plant will be built on the site of the old one. During planning, engineers quantified whole-plant electrical consumption, chemical consumption and carbon emissions.

The new facility will feature a train dedicated to stormwater treatment with a ballasted flocculation settling process, a biological treatment system using fixed-film and suspended-growth biomass, a thermal sludge hydrolysis system for improving biogas production, and biosolids drying for volume reduction.

Grease from external sources will supplement plant biosolids fed to anaerobic digestion. One-half of the dried product will be used for agriculture, and the rest will be used at a nearby cement manufacturing facility. The biosolids treatment system is to be commissioned by 2015.

Nilothi Sewage Treatment Plant, New Delhi, India

The Nilothi Phase II plant, serving a population equivalent of 455,000, treats an average 24 mgd of medium-strength wastewater. It will be the second plant serving the city, which has an existing 40 mgd plant. The new treatment line will consist of 5 mm fine bar screens, an aerated grease and grit removal system, primary settling, low-load activated sludge, secondary clarification, disc filtration and chlorine disinfection.

Primary sludge (45,700 pounds per day) will be thickened by gravity thickeners, and waste activated sludge (39,430 pounds per day) by drum thickeners. The thickened material then will be combined with grease (2,004 pounds per day) recovered from the grease and grit chamber and anaerobically digested in two 2.14-million-gallon mesophilic digesters at 20 days’ retention time.

Digested biosolids will be dewatered to 22 percent solids in centrifuges. Estimated biogas production is 257,000 scf/day (dry). The plant is projected to achieve 60 percent electricity autonomy initially, with plans to achieve 100 percent.

City of Pilsen, Czech Republic

This biological treatment plant with anaerobic sludge digestion, serving a population equivalent of 380,000, has a high organic load because of the Pilsner Urquell brewery. The plant incorporated co-digestion, adding organic wastes in the form of brewery yeast to its biosolids.

Over the last decade, plant modifications have included conversion of the digesters from mesophilic (operated at 37 degrees C) to thermophilic (55 degrees C). In addition, the sludge thickening process was optimized. From 2003 to 2010, biogas production has increased by 40 percent, and electricity production has increased by 30 percent, to 6,900 MWh per year. The plant now achieves 85 percent electricity autonomy.

South Pest Wastewater Treatment Plant, Hungary

This plant provides biological treatment and anaerobic sludge digestion and achieves 80 to 90 percent electricity autonomy. This high efficiency has been achieved mainly through a series of plant equipment replacements and digestion enhancements.

Starting in 2004, the aeration blowers, membrane diffusers and mixers were replaced with more efficient equipment. The sludge-thickening centrifuge was replaced by a gravity belt thickener, and cogeneration was employed. The blower upgrade reduced power consumption by 46 percent, from 8,880 kWh/day to 4,800 kWh/day.

The real key to approaching electricity self-sufficiency was an increase in biogas production through co-digestion using local organic wastes, including spoiled packaged food. One noteworthy challenge was that the packaged waste could not be fed directly to the digesters. The challenge was met with a process that separates the organic waste from the inert packaging material.

The system was installed, in 2007, and the result was a 20 percent decrease in electricity consumption and a commensurate increase in electricity production. Officials expect to achieve 100 percent electricity autonomy once an expansion of the cogeneration facility is completed.

Braunschweig Wastewater Treatment Plant, Germany

This plant, with biological treatment capacity for a population equivalent of 275,000, combines thermophilic anaerobic digestion, co-digestion of sludge with FOG, cogeneration using biogas-fueled engine/generators, use of biogas from a nearby landfill, and use of methane generated at a nearby green waste fermentation site. All this provides full autonomy in electricity.

Dewatered biosolids and effluent from the plant fertilize corn cropland, and, in turn, the corn is anaerobically digested. The resulting biogas is used in a cogeneration plant that produces 15 GWh electrical and 16 GWh thermal per year, providing 3,800 homes with electricity and 1,100 homes with heat.

Nakivubo Wastewater Treatment Plant, Kampala, Uganda

The Nakivubo treatment plant will have an overall capacity for a population equivalent of 380,000, treating domestic and industrial waste. The 11.9 mgd plant will include primary clarification, trickling filters, secondary clarification, and chemical addition for phosphorus removal before discharge to Lake Victoria. Sludge treatment will include anaerobic digestion, biogas recovery and cogeneration.

Achieving energy neutrality

Although the ultimate goal is net zero energy consumption, that is difficult to achieve without supplementing primary and secondary sludge with FOG, food waste, winery or brewery waste, or some other form of high-organic-content waste.

This goal is made more difficult where water-quality requirements dictate some level of nitrogen removal. Ultimately, reaching net zero energy without co-digestion may be possible for plants whose permits contain only BOD and TSS limits.

About the authors

Richard DiMassimo, P.E., is vice president of strategic planning and Brian Frewerd is vice president of engineering with I. Kruger Inc., a Veolia Water Solutions & Technologies company based in Cary, N.C.