The fundamental design and operational principles of anaerobic digestion have not significantly changed over the past two decades. However, there is a current drive to improve the performance of the existing digestion process. Major drivers that have renewed interest in optimizing this process include:

•  Requirement to cost effectively increase current capacity.

•  Demand for improved process efficiency to lower sludge treatment and disposal costs.

•  Desire to reduce the complexity of operating anaerobic digestion, including susceptibility to digester upset and foaming.

•  Pressure to meet the EPA Part 503 “Class A” standards for land application.

To address these requirements, Carollo Engineers has helped municipalities upgrade their conventional digestion systems with an emerging acid/gas two-phase anaerobic digestion.
Anaerobic digestion is a two-step process. In the first “acid formation” step, complex organic matter found in the raw digester feed sludge is hydrolyzed and acidified to produce volatile fatty acids (VFAs) by bacteria known as acidogens. The VFAs produced in the first step are the favored food of “methogenic” bacteria, which convert them into methane and carbon dioxide gas in the second step of the process. Because the optimal growth conditions for the acid-forming and methane-forming bacteria are quite different, the traditional single-tank approach is susceptible to process upsets and reduced performance.

Two-phase digestion is the logical evolution of the conventional anaerobic digestion process. Acid/gas two-phase anaerobic digestion separates the process into “acid-phase” and “methane-phase” reactors. Each is uniquely designed and operated to create an environment with the most favorable growth conditions for its specific bacteria.

The acidogens thrive in a low pH (acidic) environment, allowing them to solubilize materials that are not degraded in conventional systems. The methane-phase reactor operates at a buffered, neutral pH, which is optimal for growing the methane-forming bacteria.

•  The hydraulic retention time (HRT) is operated between one to two days.

•  The feed sludge volatile solids loading rate (VSLR) is held between 1.5 and 2.5 lb/ft3/day, and feed sludge solids concentrations are usually above 5 percent total solids.

The HRT and VSLR are controlled by varying the acid-phase digester operating volume and thickening the raw sludge feed. These operating conditions are much different than for a conventional digester, which typically operates at a 15- to 20-day HRT and VSLR of approximately 0.10 lb/ft3/day. The methane-phase digester can be run at a HRT as low as 10 days, resulting in a reduced total HRT and volume for the two-phase system. Acid/gas two-phase digestion can be operated in a mesophilic (35°C) or thermophilic (55°C) temperature range. The following variations are used in acid/gas two-phase digestion:

•  AGMM: mesophilic acid and gas phases.

•  AGTM: thermophilic acid and mesophilic gas phases.


•  AGMT: mesophilic acid and thermophilic gas phases. As with other thermophilic digestion systems, AGMT systems are used to exceed the Class A pathogen land application criteria.
Two-phase digestion addresses the four main drivers for improving municipal solids treatment.

Cost-effective capacity increase – Based upon the following attributes, the overall reactor volume for a two-phase system is much smaller than a conventional single-phase system with equal treatment capacity.

•  Separation of the digestion phases allows the biological conversions to occur more rapidly than in a conventional system.

•  The ability of the acid formers to thrive at a higher solids loading rate allows the incoming sludge to be thickened to as high as 8 percent total solids, which is nearly double the allowable thickness of a conventional system.

Lower sludge treatment and disposal costs – The improvement in process efficiency by using acid/gas two-phase digestion impacts the sludge treatment and disposal cost in three major ways.

Greater solids destruction – Conversion to a two-phase digestion system typically increases volatile solids destruction by 20 percent or more. This reduces the amount of digested solids that must be dewatered and/or disposed of.

Improved digested sludge dewaterability – Polymer cost is reduced by up to 25 percent because there are less solids to dewater. Also, dewatering system performance can be increased because the structure of the biological solids is more completely cleaved.

Enhanced methane gas production and quality – Methane gas production typically increases by 20 percent, primarily due to increased volatile solids destruction. The gas also has a higher energy con-tent and is less corrosive because the “sour” gas of the acid-phase digester is isolated and destroyed.

Fewer operational problems, including the susceptibility to digester upset and foaming – Upsets in conventional anaerobic digesters are usually attributable to the methogenic bacteria, which are difficult to grow and sensitive to overloads. Two-phase digestion is resilient to changes in feed volume and composition because the acidogenic bacteria are very hardy and actually thrive under extreme loading conditions.

The combination of the acid-phase digester environment with low gas production also minimizes or eliminates foaming problems. In fact, the objective of the first major conversion from conventional to two-phase digestion at the Woodridge Wastewater Treatment Plant in Dupage, Illinois, was to control digester foaming.

Improved pathogen destruction – Two-phase digestion can be easily converted to a thermophilic process capable of producing a disinfected end product. Ongoing research has also indicated that a completely mesophilic acid/gas two-phase digestion system can potentially meet Class A standards.
Cities of Reno and Sparks, Nevada – To increase digestion capacity and significantly reduce costs, Carollo designed an acid-phase digester and converted four existing digesters to methane-phase operation at the Truckee Meadows Water Reclamation Facility.

City of San Bernardino and Inland Empire Utilities Agency (IEUA), California – In two separate projects, Carollo helped these clients convert their facilities from conventional digestion to two-phase digestion, resulting in a 35 percent decrease in bio-solids disposal requirements at both plants.

City of Phoenix, Arizona – Carollo analyzed six digestion alternatives and designed a retrofit of a conventional anaerobic digestion process to an AGMT process at the 91st Avenue Wastewater Treatment Plant.

City of Turlock, California – This project involved designing an acid-phase digester to break down solids prior to processing with conventional methane digesters at the Turlock Water Quality Control Plant.