Anaerobic Digestion

Anaerobic Digestion provides required solids stabilization to support the use of biosolids and produces renewable methane.

Anaerobic digestion is a solids stabilization process in which biodegradable organic matter is biologically transformed in the absence of oxygen. This process provides a reduction in pathogens, reduces vector attraction, and generates renewable methane. 

Thickened primary sludge (TPS) and concentrated waste activated sludge (CONWAS) have a high volatile fraction and contain pathogenic microorganisms. Treatment is needed to produce a safe fertilizer product with minimal odors while not attracting insects and other pests. The objectives of stabilization are to reduce pathogens, eliminate offensive odors, and inhibit, reduce, or eliminate the potential for putrefaction (decay or rotting). 

The primary benefit of anaerobic digestion, compared to other forms of solid’s stabilization, is the energy recovery potential. Anaerobic digestion generates a biogas comprised primarily of methane and carbon dioxide that may be recovered for beneficial use such as heat or power generation, or with further treatment turned into a renewable natural gas. 

The RWHTF facility utilizes a two-phase anaerobic digestion process.

  1. The undigested, or raw sludge, from holding tanks goes to a single digester called the acid-phase digester, which is used as the hydrolysis, or initial breakdown, phase of the digestion process. The detention time in this digester is typically one to one and a half days
  2. In the second stage, commonly referred to as the mesophilic phase or gas phase, a minimum of 15 days solids retention time (SRT) is required to meet the objectives of the anaerobic digestion treatment process. 

Northern Treatment Plant

The NTP also uses anaerobic digestion to stabilize its solids, but only uses a single-phase anaerobic digester, skipping the acid phase digestion step.

The digestion processes must meet operational standards as listed in 40 CFR 503 and 5 CCR 1002-64. These operational standards include pathogen destruction criteria and vector attraction reduction. These compliance requirements are met in the anaerobic digesters. 

For pathogen destruction criteria, both RWHTF and NTP use processes to significantly reduce pathogens (PSRP), specifically anaerobic digestion. The sewage sludge is treated in the absence of air for a specific mean cell residence time at a specific temperature. Values of the mean cell residence time and temperatures must be kept between 15 days at 35°C and 60 days at 20°C. RWHTF averages 19.5 days SRT at 38°C. NTP averages 54 days SRT at 36°C. 

At RWHTF, during periods when digesters are out of service for maintenance, an alternative method for proving pathogen destruction criteria is completed in addition to the time and temperature requirements. During these months, seven representative samples are taken to represent the month for fecal coliform. The geometric mean of those samples must be less than 2,000,000 most probable number per gram of total solids (MPN). 

To meet the vector attraction requirements, the mass of the volatile solids in the sewage sludge must be reduced by 38 percent. This is demonstrated by testing the volatile solids in the sludge feed entering the digester and leaving the digester to find the reduction. 

Capital Investments

None active

Robert W. Hite Treatment Facility

Capital Investments (15 years)

Projects are based on the current anticipated regulatory requirements but are subject to change without notice. Projects are planned and are subject to change without notice including the cost estimates.

Digester Mixing and Covers

2028-2035 Investment: $136M

Improvements to the mixing of the mesophilic digesters to improve VSS destruction and biogas production.

Treatment Process Map Learn more about biosolids Learn more about the collection system Learn more about the influent Learn more about the sources