Clean-water plants looking toward energy self-sufficiency often seek more biogas output from their anaerobic digesters. Some get it by adding food waste and fats, oils and grease to the primary and waste activated sludges.

CNP - Technology Water and Biosolids Corp. has another approach to gas boosting in its PONDUS thermochemical hydrolysis process. In a reactor upstream from the digesters, hot water and caustic soda are used to break the cell membranes in waste activated sludge. The hydrolyzed material is then fed to the digesters along with primary sludge. Being more readily digestible, this material enables the digestion process to increase gas production substantially. In addition, the process makes the resulting biosolids easier to dewater and with less polymer.

CNP recently commissioned its first PONDUS installation in the United States at the wastewater treatment plant in Kenosha, Wisconsin. Gerhard Forstner, company president, talked about the process in an interview with Treatment Plant Operator.

TPO: What was the market need behind development of this process?

Forstner: The main motivation was to increase biogas production, and a very close second was to improve dewaterability. Later we discovered the potential for reducing polymer consumption and improving digester loading rates. The process came out of German technology that was developed in the mid-2000s.

TPO: What drove the interest in greater biogas production?

Forstner: In the municipal marketplace in Europe, plants larger than 2 to 3 mgd typically use anaerobic digestion to produce biogas to fuel combined heat and power units. They could produce electricity probably for 12 cents per kWh, and they could buy utility power for about the same rate. So it made economic sense to break even producing electricity while producing free heat for the digesters, or to dry biosolids. Another possibility was to use hot water to hydrolyze the solids. That drove the development of the PONDUS technology.

TPO: Is this technology a thermal process?

Forstner: Not entirely. It combines caustic soda — sodium hydroxide — for pH change along with hot water. That combination enables us to hydrolyze the sludge efficiently.

TPO: Where in the solids process is this technology deployed?

Forstner: Upstream of the anaerobic digesters. In the digestion process, there are two main steps. The first is hydrolysis, where the cells in the waste activated sludge are broken down. That typically takes seven to 10 days in the digester. Then the next step is the production of methane. So what we do is take the hydrolysis step out of the digester. We predigest the food in a reactor before it goes into the digester. That makes the digester more efficient because all the bugs are immediately focused on producing methane.

TPO: Does this process work on both primary and waste activated sludges?

Forstner: The process uses only waste activated sludge, because that is the sludge that is much harder to digest. We then blend the hydrolyzed material with primary sludge and feed the mixture into the digester at about 100 degrees F.

TPO: How exactly does the hydrolysis process work?

Forstner: We first thicken the waste activated sludge to 7 to 10 percent solids. We then dose that with about 1,500 parts per million of 50 percent caustic soda. Next we mix one part of that fresh sludge with two parts of hydrolyzed sludge recycled from the PONDUS reactor. That reduces the viscosity by 80 to 90 percent. This material passes through a heat exchanger and enters the reactor.

TPO: What happens inside the reactor?

Forstner: It’s a circular reactor built for two- to 2 1/2-hour detention time depending on the characteristics of the incoming material. About 10 inches inside the outer reactor shell is a smaller ring. We feed the sludge at the lowest section of the reactor, about 5 inches from the bottom, between the inner and outer rings.

The sludge rises toward the top of the reactor bed and is heated to 150 to 170 degrees F. After about 1 1/2 hours, it overflows from the outer ring into the inner ring, where there is another half-hour of detention time. During this process, the caustic soda disintegrates the cell membranes, releasing organic acids and neutralizing the pH from 11 to 7. After another half-hour, the hydrolyzed sludge is withdrawn from the bottom of the reactor. Heat from the hydrolyzed sludge can be captured to heat the digesters.

TPO: What is the impact of this process on the anaerobic digestion phase?

Forstner: The material sent to the digester is totally different from waste activated sludge. It is mixed with primary sludge before feeding to the digester. Most of the food is predigested, and methane production can start immediately. That means we can have reduced detention time in the digesters, or we can load the digesters with more solids. We get higher volatile solids reduction; lower organic content means we get higher dewaterability and drier cake solids.

TPO: What is the effect on biogas production?

Forstner: We typically increase biogas production by 20 to 25 percent. We also reduce polymer consumption by 15 to 20 percent, and the solids cake is drier by 3 to 5 percentage points. Energy consumption is 0.9 to 1 kWh per cubic meter of sludge. The payback time for a system is typically three to five years.

TPO: What experience does this technology have in full-scale commercial operation?

Forstner: There are seven installations in Europe; the first was installed in 2007. The process is installed in plants as small as 5 mgd and as large as 150 mgd.

TPO: Are there any other potential benefits to this hydrolysis process?

Forstner: It can be combined with phosphorus recovery. You can use a filter or other device to separate the solid and liquid phases of the hydrolyzed material. The liquid phase is rich in phosphorus that can be recovered as struvite. The system can also be configured to achieve Class A biosolids.