Biosolids digester replacement project delivers savings and helps sustainability

By Kevin Litwiller and Sarah Mason-Renton

Located between Lake Erie and Lake Huron, the city of St. Thomas, Ontario is a growing community with a current population of nearly 40,000. Like many smaller cities, St. Thomas had historically used anaerobic digesters as part of its wastewater treatment process. Biosolids from them were transported to the local landfill, which meant there was no opportunity for resource recovery. Despite maintenance and repair efforts, the city’s aging wastewater treatment plant’s digesters had begun to fail, which meant a new plan was necessary.

These factors, combined with pending regulatory changes, as well as the city’s desire to have a sustainable, long-term solution, were reasons to find a better way to manage its biosolids.

In partnership with XCG Consultants Ltd., the city’s project evaluation team considered two alternatives. The first was an anaerobic digestion process, which required upgrading the failing digesters and implementation of an energy recovery process. The second was Lystek’s THP® process to produce CFIA registered biofertilizer that could be sold to the local market.

In the final analysis, it was the clear economic benefits of replacing the failing digesters with a low temperature, patented and proven system that could recover resources to produce a valuable, federally registered end product that resulted in a recommendation.

The Lystek approach offered significantly lower lifetime capital and operating costs than any of the alternative, short-listed options. In fact, it was determined through a detailed Net Present Value analysis that the solution would be able to provide the city with 40% reduction in cost savings. This solution would help the city to develop and implement a sustainable, cost-efficient, long-term biosolids management program, with capacity to service future growth.

The process is a simple and reliable solution that hydrolyses dewatered biosolids, using low-temperature steam, high-speed shearing and alkali, in an enclosed, low-pressure reactor. The project for St. Thomas also included on-site tank storage for up to nine months of biofertilizer product. The agreement also provided the city with turn-key management of the end product, including marketing, sales and best practice field application of the CFIA registered fertilizer. All of these considerations were factors in the final decision to move to a true resource recovery model.

Unique approach

This solution offers a unique approach to biosolids management. Whereas this process has previously been added to wastewater treatment plants to augment, and even optimize, existing processes and enhance resource recovery, this is the first project where plant operations were fundamentally altered around the low temperature Lystek THP process. In the case of St. Thomas, the smaller size of the plant, aging infrastructure and capital constraints made replacing the digester the most cost-effective solution for the municipality, while also facilitating the transition to a resource recovery centre.

Converting biosolids an environmental consideration

In addition to significant cost savings and technical merit, environmental considerations were another major factor in the decision. The project evaluation team noted that the process is “highly beneficial” through its conversion of processed biosolids into fertilizer and reuse of the large quantity of available nutrients.

Biosolids typically contain significant amounts of valuable nutrients which are lost in landfills. However, the agriculture sector understands the need for high-quality nutrients and organic matter to promote robust plant growth and soil health. The performance of LysteGro® biofertilizer has been repeatedly demonstrated through third-party field trials and repeat orders.

Another benefit is the system’s ability to produce a product that meets, and exceeds, the toughest regulations in North America, which relate to the reduction and elimination of pathogens, while retaining essential macro- and micro-nutrients and critical organic matter. Instead of being sent to landfill, biosolids are now processed into a federally registered, Class A product that can be sold and utilized in the local market.

A well-designed odour control system and quality-controlled approach to processing were other critical points. The evaluation team factored in Lystek’s experience and approach to control and mitigate odours with a sealed, end-to-end processing, storage, transfer and end-product use system.

Simple and proven

Originally developed in Ontario in 2000, the patented Lystek system is now being utilized by a rapidly expanding range of small, medium and large communities throughout North America. This includes cities and major metropolitan areas as large as Toronto and San Francisco, and smaller centres such as Centre-Wellington (pop. 28,000) and Guelph, Ontario (pop. 129,000). Some of these sites have been operating successfully for ten years.

Lessons learned

With low temperature thermal hydrolysis leveraged to replace, rather than augment, anaerobic digestion, higher levels of physical contamination can be present in raw wastewater. In these cases, a pre-processing step, such as supplementary grinding as well as maceration prior to thermal hydrolysis may be required to ensure the quality and integrity of the end output.

Additionally, the dewatering and processing of raw wastewater and waste activated sludge can stress potential hydrogen sulfide levels released during dewatering. Understanding and identification of this possibility needs to occur early in the analysis so it can be factored into both the evaluation and design stages of the project.

Conclusion

The St. Thomas project demonstrates that replacing anaerobic digestion with the Lystek THP process can be a cost-effective solution for small to medium sized facilities with capital constraints and aging or lack of digesters. This is particularly true where the scale of potential energy production is not sufficient to make optimal use of the biogas produced. This solution has been successfully implemented to address the St. Thomas’ legacy infrastructure challenges and solids management requirements, while still prioritizing resource recovery and nutrient capture.

Kevin Litwiller, BBA, and Sarah Mason-Renton, PhD, are with Lystek International. This article appears in ES&E Magazine’s October 2018 issue.