Microbiologists and chemical engineers at a Cardiff-based biotech company have developed an economical, low carbon footprint solution to optimise the anaerobic digestion process. This solution decreases the volume of undigested sludge and increases the biogas yield, improving the AD circular economy.
Anaerobic digestion (AD) is fast becoming an integral part of the wastewater treatment process for local authorities and private companies. The ability to use microorganisms to produce methane rich biogas from sewage sludge and organic waste matter offers the potential to produce heat and electricity from an ever present and growing waste source. Through the AD process, specialised microbes interact in the absence of oxygen to breakdown the organic components of sludge and waste to facilitate biogas production.
The AD system therefore greatly decreases the final volume of the original sludge or waste to be disposed of whilst yielding energy that can be used to offset plant processing and operating costs. Furthermore, the final waste or digestate produced from the AD process is capable of being used as a bio-fertiliser in what is termed sludge to land strategies.
Genesis Biosciences developed the Evogen Biogas Additive at its South Wales laboratories and it is the latest product in the company’s range of microbial solutions. The company’s experts identified a novel mineral carrier with the correct parameters to bolster the most sensitive microbial components of the AD system. Through extensive laboratory and genomic screening, it was discovered that if the carrier was paired with non-pathogenic and naturally occurring Bacillus microorganisms that are capable of anaerobic growth, it significantly enhanced sludge and organic polymer degradation.
Simple biotech solution Together, the two components offer a simple to apply biotech solution that can operate over a wide span of pH values and temperatures in a range of reactor configurations. The expansive metabolic capabilities of the Bacillus microorganisms and the universal tolerance of the mineral additive allow application across most organic feed types spanning many different industries including sludge fed systems, agricultural waste fed systems, slurry fed systems, food and municipal fed systems, landfill leachate systems and one and two phase configurations.
Dr Chris Charles, senior research scientist and wastewater treatment specialist at Genesis Biosciences, explains: “With a growing demand on companies and authorities to better their environmental credentials, decrease their waste and lower their CO2 emissions, AD and practices such as sludge to land are now being touted as circular economy champions.
“The ability to take human and other waste streams such as agricultural and food, and turn it into both energy and a fertiliser is attractive from a commercial aspect and a palatable ‘green’ concept that the general public can both understand and appreciate.”
Although high in potential to create a beneficial circular economy, there are still some key challenges and drawbacks with the AD system including time, waste, yield and costs. The process can be slowed down by inhibitory environmental conditions that impact the resident microbial consortia and lead to a decrease in metabolic activity.
Low microbial activity, less than ideal growth conditions or an inefficient AD system can also cause losses in methane yields and therefore lead to high volumes of residual sludge which must be processed and disposed of via practices such as landfill or incineration. All these downsides increase the running costs and ultimately impact the cost-effectiveness and environmental credentials of AD systems.
Optimising production The Evogen WWT Biogas Additive helps to optimise the anaerobic digestion process and overcome these common challenges. Working in tandem, a mineral-based powder carrier and Bacillus microbes assist in the breakdown of organic feedstocks to optimise biogas production. The mineral-based powder carrier acts as an ion exchanger that boosts interspecies electron transfer and serves as a colonisation site for methanogens, while the Bacillus microbes enhance the hydrolysis and fermentation phases of the digestion process by secreting anaerobically active hydrolytic enzymes.
Dr Charles continues: “Following the addition of the Evogen Biogas Additive, the biogas conversion rate is accelerated, the volume of undigested sludge is reduced, and the biogas yield is significantly increased, resulting in lower running costs and a greater financial return. By partnering interacting Bacillus microorganisms that possess diverse extracellular enzyme profiles, we can ensure that our additive performs across a broad spectrum of organic substrates from sewerage sludge to agricultural and food wastes. “In combination, both the mineral carrier and the Bacillus microorganisms form a stable product and boast a long shelf life of up to and over two years. Which is considerably higher than other sensitive types of microorganisms that could be used as a potential additive.”
Case study A single stage wastewater treatment (WWT) sludge fed system was dosed with Evogen Biogas Additive over a three-month period. The plant received 240m3 of WWT sludge per day with 10% of that volume coming from landfill leachate. It operated at pH 7 and 35°C with a residence time of 10.4 days. A boiler was used to heat the sludge which was powered by both diesel and biogas. The sludge was disposed of via landfill after thickening using a flocculent and belt press. Evogen Biogas Additive was dosed at 0.15% of the dry matter inlet concentration, with an initial slug dose of 50 kg followed by a daily dosage of 10 kg. Dosing was performed on site using a small, modified tank to create a slurry with influent and which was then pumped into the system before the heat exchanger
Results Dosing of Evogen Biogas Additive over the three-month period was able to significantly reduce final sludge volume by half and double biogas production consistently during the trial period. This resulted in significant cost savings from less boiler diesel usage, lower flocculant use and lower sludge disposal costs. The overall cost saving over the trial period totalled just over €215,000.
Overall, Evogen Biogas Additive was used as a cost effective and simple to apply tool to not only improve plant efficiency but also to positively impact the environmental parameters of the plant by reducing sludge production and diesel consumption.
Disposal fees and amounts associated with final sludge were greatly de-creased whilst energy and heat gener- ation from biogas were greatly improved, demonstrating the capability of Evogen Biogas Additive towards the benefit of a circular economy approach. Evogen Biogas Additive is supplied in Italy by Bierrechimica and Tillmanns Spa.