Higher-quality biosolids that meet U.S. EPA Class A criteria can be reused nearly without limitations as a fertilizer to promote productive soils and stimulate plant growth.

Anaerobic digestion upgrades that enable clean-water plants to produce Class A material can offer multiple benefits, such as reducing the volume of biosolids so less product needs to be managed and transported.

One such upgrade is biological hydrolysis technology, which has been used for preconditioning sludge since 2002, mainly in the U.K. The technology can be installed as a retrofit ahead of existing anaerobic digesters to enhance digestion efficiency.  

Beginning in 2016, SUEZ Water Technologies & Solutions began a program to alter the biological hydrolysis process to further optimize digester performance and produce a Class A product. The goal was to create an alternative to thermal hydrolysis — and other technologies that yield Class A biosolids — that would achieve similar performance for a lower capital investment and using less sophisticated equipment.

Boosting the rate

It is widely accepted that hydrolysis is the rate-limiting step of anaerobic digestion. To adequately stabilize sludge, extended hydraulic retention times are commonly designed into the digester volume.

Biological hydrolysis addresses this rate-limiting step by adding six serial reactor vessels in front of the digesters. It increases digester capacity by two to three times by reducing the required retention time and providing optimum conditions to maximize the hydrolysis rate. The process also helps boost biogas yield and reduces biosolids volume.

The most recent biological hydrolysis solution includes a design hold time of five hours and a pasteurization step that elevates the temperature to 131 degrees F in the last three reactors. While this achieves an “enhanced biosolids” designation in the U.K., it does not meet EPA Class A requirements.

Upping the ante

To achieve the Class A designation, the next step was to target the EPA 40 CFR Part 503 rule, specifically Alternative 1: Thermally Treated Biosolids. This is one of six defined alternatives for achieving Class A; it sets a requirement based on hold time at a certain temperature.

Adapting biological hydrolysis technology to meet Alternative 1 required a hold time of 24 hours at solids content less than 7% and 63.1 hours for solids content above 7%, based on the 131 degrees F hold temperature in the last three reactors. Although both scenarios were far above the five-hour design hold time, the six-pack of tank reactors provided a platform for altering process conditions to meet Alternative 1 without changing the physical characteristics or the size of the system. 

Validating the design

At first glance, it would appear relatively straightforward to redesign the biological hydrolysis technology into a Class A solution: By adjusting the pasteurization temperature, the process flow could be configured to match or exceed the time-temperature requirements.

But because the core purpose of biological hydrolysis is to increase digester effective capacity and efficiency, a new Class A hydrolysis process would have to prove there was no detrimental effect on overall performance.

To that end, a performance model was developed to compare biological hydrolysis digestion combined with mesophilic anaerobic digestion against conventional mesophilic digestion alone. The model showed that adding biological hydrolysis enhanced digester performance by reducing volatile solids when compared to pure mesophilic digestion. Next, the performance model was validated by comparing it against full-scale operating data from eight active biological hydrolysis plants, establishing a performance benchmark.

Stepping up to scale

The next step was to demonstrate the process at full scale by constructing a system with a six-tank biological hydrolysis system and mesophilic anaerobic digestion tank at the City of Guelph Wastewater Treatment Plant in Ontario. The demonstration system was built next to an existing mesophilic anaerobic digestion facility, enabling easy performance comparison.

Results showed that the demonstration system, operating at a digester retention time of 12 days, reduced volatile solids by 52%, whereas in the anaerobic digestion facility, the volatile solids reduction was 46% at a digester retention time of 22 days.

This documented a significant improvement in the overall digestion process by adding biological hydrolysis, even with a different digester retention time. When the demonstration system performance was adjusted to a 20-day retention time, the resulting volatile solids reduction was 58% — a 26% improvement in digester performance. In addition, demonstration system operated in a scenario consistent with the Class A time-temperature requirements of EPA Alternative 1. 

Besides meeting the time-temperature requirements, the biosolids must be below prescribed pathogen concentrations to qualify as Class A. Testing revealed that the biosolids conformed to fecal coliform limits and that Salmonella was nondetectable. 

Finalizing a commercial solution

The Class A hydrolysis solution has now been commercialized, and as of May, the first two projects using the revised biological hydrolysis technology were being finalized. To enhance the effectiveness of Class A hydrolysis, a thermal drying technology can be added to further reduce biosolids volume and boost savings on biosolids management and transport.

About the author

Michael Theodoulou (michael.theodoulou@suez.com) is a senior product manager with SUEZ Water Technologies & Solutions.