Aug-2024

Waste to revenue for meeting decarbonisation goals

Technology enhancements that recover energy from waste provide multiple benefits, including cost reductions and the creation of new revenue streams.

Dave Swerdlyk
Veolia Water Technologies & Solutions

Article Summary

Wastewater treatment plants (WWTPs) can play a larger role in contributing to decarbonisation goals. Today, more technology options are available for them, which can ultimately lead to reductions in greenhouse gas (GHG) emissions. However, the options are not solely driven by climate-related benefits. Also in the mix are substantial economic returns, including cost reductions and the creation of new, long-term revenue streams to support operations beyond traditional user rate financing.

The strategies for achieving these results primarily revolve around the management of organic waste and sludge, the latter a byproduct of wastewater treatment. When these materials are landfilled, they decompose over time and release GHG emissions, primarily carbon dioxide (CO2) and methane. CO2, the predominant GHG from human activities, is the largest contributor to global warming. Methane ranks as the second most prevalent human-made GHG, responsible for approximately 16% of global emissions. Notably, methane concentrations in the atmosphere have more than doubled over the last two centuries. The US Environmental Protection Agency (EPA) notes that methane is more than 28 times more potent than CO2 at trapping heat in the atmosphere.

The EPA also identifies municipal solid waste (MSW) landfills as the third-largest source of human-related methane emissions in the U.S., highlighting the critical role of waste diversion strategies. This is where WWTPs can make a difference. By treating sludge and organics with anaerobic digestion (AD) technology, WWTPs can reduce the volumes of waste destined for landfills, reducing trucking and disposal costs, lowering GHG emissions, and extending the life of these landfill sites.

The added value of AD is the opportunity for resource recovery. The AD process generates methane-rich biogas as a byproduct, which can be captured and utilised in multiple ways. One method involves using it to power a combined heat and power (CHP) system, generating both electricity and thermal energy for on-site use. An even more profitable approach involves upgrading the biogas into renewable natural gas (RNG) and injecting it into a commercial natural gas pipeline. This allows the RNG to be transported and sold in the energy market, where a higher payback can be realised as it can be monetised in a variety of transportation, heating and commercial applications.

The sale of RNG provides a commodity value. Additionally, under the Federal Renewable Fuel Standard (RFS) Program, RNG producers can earn renewable identification number (RIN) credits. These credits, used to track renewable fuel production and usage, can be traded on the market and serve as the currency of the RFS program, offering another revenue stream. Companies that do not meet the mandated renewable fuel amounts (referred to as ‘obligated parties’, such as petroleum refiners and importers of refined fuels) must buy these credits to meet regulatory requirements.

In addition to reducing GHG emissions from landfills, converting sludge and organic waste into RNG is a beneficial circular economy practice that offsets the use of fossil fuel-derived natural gas, lowering the carbon footprint of energy production.

Boosting biogas recovery
Since AD – by recovering biogas – offers a pathway to a financial return, any technology that can optimise the AD process should be considered as a potential strategic investment. This rationale underpins the adoption of biological hydrolysis (BH). Integrated immediately upstream of the AD system, BH technology is designed to condition sludge to accelerate the efficiency at which it can be digested, thus boosting biogas production. By removing the rate-limiting step (hydrolysis phase) from the digester, the 20-30-day hydraulic retention time (HRT) required for conventional mesophilic digestion is reduced to 15 days, significantly increasing the throughput of the digester.

BH technology controls conditions in a multi-stage series of flow reactors to create an optimal environment for hydrolysis and acidification, promoting the development of volatile fatty acids to feed the digester. This allows the digestor to be dedicated to methanogenesis, effectively increasing its capacity. The parameters of the BH flow reactors can also be adjusted to reduce the volume of produced biosolids while improving their quality to yield a pathogen-free biosolid. This higher quality biosolid product can be reused beneficially as a soil improver to help promote productive soils and stimulate plant growth. Critically, the BH + AD combination can achieve greater GHG emission reductions by realising 100% waste diversion from landfills.

As a proven technology for pretreating sewage sludge prior to digestion, the BH process enables several benefits: reduced retention times in digesters, improved sludge digestibility, enhanced sludge stability to minimise foaming in the digestion plant, as well as increased volatile solids (VS) conversion in the digestion system. Additionally, BH is compliant with US EPA 40 CFR Part 503 regulations for Class A biosolids treatment.

Making the economic case for BH and AD
Traditionally, BH technology applications have been limited primarily to the UK. This is set to change with the completion of the University Area Joint Authority (UAJA) biosolids upgrade, a project that will mark the first North American installation of BH technology.

The UAJA, located in State College, Pennsylvania, operates the Spring Creek Pollution Control Facility. This facility is situated on the boundary of Benner Township and College Township in Centre County, Pennsylvania, and provides wastewater treatment, solids handling, and water reuse services to the Centre Region. It also includes an existing composting process that converts sludge and wood chips into a Class A biosolids compost product, which is sold nationally. This composting operation meets the US EPA’s Biosolids Rules and Regulations as a Process to Further Reduce Pathogens (PFRP). Additionally, the sewage sludge processed here meets vector attraction reduction (VAR) requirements, qualifying it as Class A biosolids.

Operational since 1993, the original composting facility was nearing the end of its useful life, prompting UAJA to enlist engineering consulting firm RETTEW to design an upgraded solution. UAJA evaluated a multitude of options, narrowing these down to two viable alternatives: expanding the existing composting facility or replacing it with a comprehensive treatment system featuring BH, AD, and sludge drying.

RETTEW conducted an economic analysis of both options, evaluating the relative capital and operating costs over the next 25 years. Those calculations are shown in Table 1.

RETTEW’s economic analysis included several assumptions such as costs of natural gas of $7/MMBTU, electricity costs of $0.075 per KWHr, RNG revenue, inclusive of environmental attributes such as RINs, of $15/MMBTU, Class A biosolids revenue of $10/ton, and tipping fees of $35.00/ton of waste. Based on the clear technical advantages and especially the projected economic 25-year cost benefit of BH, AD, and sludge drying, UAJA selected the second alternative to address its long-term handling and disposal of wastewater treatment plant biosolids.