Introduction to Anaerobic Digestion

Basics of Anaerobic Digestion

Wet Digestion

The wet anaerobic digestion process is applied to liquid waste streams that are conveyable by liquid pumping. Sometimes wet systems are called Low Solids AD (LSAD). The Wet AD process can be done in reactors of two main configurations, continuously stirred tank reactors (CSTR) and plug flow reactors. The theory of the CSTR is that, through rigorous mixing, the composition of the contents of the reactor in any given spot in the tank is the same as in any other spot in the tank. The theory of plug flow, on the other hand, is that the makeup of the contents at the head of the digester is different than that of the material leaving the digester just as the material flows through the digester in the pattern like a plug through a pipe. Wet systems commonly run at total solids levels between 2 and 8 percent. Wet systems will often start with a liquid manure or waste biosolids as the backbone of their feedstock load to provide a baseload buffering affect for enhanced process stability.

A key design parameter for any digester system is the overall loading rate. For any given project no two digester suppliers will provide a system of exactly the same size. Loading rates are commonly expressed as the number of days of retention time or the quantity of organic matter applied to a given tank volume. Common detention times for farm based manure digesters are roughly 20-30 days. Experience has shown that this time represents an optimum time where gas yield is maximized without over designing the residence time. Facilities that are co-digesting more complex wastes that include fats and proteins will commonly have retention times higher than 30 days.

The wet AD process is commonly designed at one of three different temperature zones phytotrophic, mesophillic and thermophilic. Each temperature zone relies on a different species of bacteria that flourish at those given temperatures.

Temperature Regime Degrees F Degrees C
Phsychophilic 60-75 15-20
Mesophilic 95 30-40
Thermophilic 120 50-60

The choice of which temperature zone to operate can be a function of the available feedstock, project site logistics, costs for heating and the end use of the digestate. Higher temperature systems can provide additional pathogen kill efficiency compared to lower temperature systems. That additional efficiency comes at the price of needing to apply more energy to the digester to provide proper heat, however. The lower temperature, mesophillic systems can provide the benefit of a faster growing, more robust bacteria population vs thermophilic which have slower growing bacteria. If virus or pathogen kill is a concern sometimes a separate heat treating, or pasteurization step can be used as pretreatment to enhance kill effectiveness.
A process known as TPAD (Temperature Phased Anaerobic Digestion) is a combination of both thermophillic and mesophilic treatment in stepped reactors in series and is commonly applied at municipal applications. The first stage thermophilic treatment provides needed pathogen kill effectiveness followed by the second stage mesophillic treatment that gives the bulk of the biogas formation and solids destruction.

Heating Systems
Digesters need to be kept at a steady, warm temperature for optimum gas yield and stable system operation. Ideally digester heat requirements can be met by capturing waste heat from other facility unit operations, such as a Combined Heat & Power (CHP) system. Sometimes, such as during start up, supplemental heat from a gas boiler is required to get a system up to operating temperature. Heat can be applied internally to the tank through a series of hot water pipes that are either embedded in concrete tank walls or supported on the inside tank walls.

See link for biogas heating video:

Heat can also be applied externally to the tank from an external heat exchanger where contents from the tank are heated in either a shell-in-tube or plate heat exchanger using a heat transfer fluid, usually water, that pulls heat from the CHP process that has waste heat and moves heat to the tank contents.

Bottom Grit removal systems
The ideal digester scenario is one that never needs to be taken out of service during their long, useful life. It is therefore important to be able to remove settled grit that can accumulate over time. Systems that employ a rotating scraper arm across the tank bottom, for one, can be used to force contents to the tank wall where they can be properly removed. Tanks can also be installed with sloped bottoms that force contents to the sidewall or tank middle for removal. The optimum pretreatment system will keep as much grit as possible out of the tank ahead of time but there’s no way to keep it all out so it is important to remove grit to keep 100% of the tank’s usable digestion volume functioning. Unfortunately, applications where there are heavy loads of grit or interts in the feedstock can require the tank be taken out of service every few years if the level of inerts are allowed to build up on the tank bottom.

Floating Layer
In digester applications there can be a tendency for oily and fatty compounds to float out of solution and rise to the top of the digester’s contents. If there is not adequate mixing to keep tank contents completely agitated this floating layer can accumulate to significant thickness if not properly managed. The best approach to managing a floating layer is to properly engineer the mixing system ahead of time to ensure proper agitation to prevent its formation. Some gas mixing systems have the ability to break up this floating crust layer one it has been formed.

Pressure and Flammability Considerations
Safety is a paramount concern for anaerobic digester owners. A properly designed system can operate reliably and trouble-free for decades. Pressure swings in the tank should be carefully monitored and designed for. Biogas that is generated in the process causes pressure to build up in the tank head space. Typical pressures range from 4-16 inches of water column (there are 27.7 inches of water column in one psi). Common safety design measures include a pressure relief valve, a flame arrester and a vacuum breaker. The pressure relief valve can be set at a pressure that is well below the point that would cause serious digester equipment damage. On a tightly sealed digester there would often be a burst disc installed as well. Vacuum breakers are installed to accommodate the potential for downstream gas uses that would include fans or blowers that pull on biogas to send it downstream for use. Such fans can pull vacuums internally inside the tank and cause internal stress from the negative pressure.

When manures are a majority of the feedstock, designers will commonly apply wet digester technology. Because manure has a relatively modest potential for generating biogas (the cow has already done most of the digestion) it can be economically advantageous to add additional substrates to boost gas yield. Wastes such as brown grease, DAF floats, cheese whey, and commercial food wastes can boost gas generation potential. Overloading a digester can be detrimental to performance, however. An upset digester, behaves much like an upset digestive tract in a human in a way. While digesters don’t get “heartburn”, per se, one will see system pH drop when Volatile Fatty Acid (VFA) production is rampant. At times like this it is important to back down on feed rates to let the system recover. Gas yield often drops off measurably when the system is upset from an overload condition.

CSTR Design
The main features of the complete mix, or CSTR, design include the tanks, mixers, covers and heating systems. Some CSTR designs use a single mix tank where all phases of treatment (hydrolysis, methanization, etc) happen in a single tank. This tank would sometimes have a flexible membrane cover to store resulting biogas and would utilize mixers that mount on the side of the tank and penetrate into the tank generally with a propeller or mixing jets to mix the tank’s contents. Some CSTR systems will split treatment into two tanks running in series. This style accomplishes hydrolysis and acidification in the first tank and the methanization in the second tank. This style is commonly noted by a first stage with a fixed roof with a roof mounted, high torque, slow speed propeller style mixer mounted from the top. The second stage methanization tank will be noticeable by its domed shape flexible gas cover as shown in the picture below.

CSTR Mixing Systems
There are three chief means of mixing a CSTR digester; by gas, mechanical or hydraulic mixing techniques. Gas mixing systems operate by compressing biogas and injecting it into the the tank contents. Resulting rising gas bubbles dislodge the contents either in the bottom or top of the digester and keep them stirred. There are two types of mechanical mixing systems, those with their moving parts in the basin and those with their moving parts outside the basin. The former commonly use either side mounted propeller style mixer and a hoist mechanism to facilitate easy motor maintenance or a roof mounted propeller. The latter will commonly employ mixing jets where tank contents are first pulled out of the basin by a motive pump and reintroduced at a high velocity back to the tank through a jet nozzle to keep contents stirred and agitated. Properly designed mixing systems will commonly “spin” the tanks contents on a vertical axis. Common design consideration and operational issues to watch are the potential for grit accumulation on the tank bottom and the possible accumulation of a floating fat or scum layer on the top of the tank. Well designed mixing systems will be able to keep bottom solids agitated and top floating layers “folded in” to the mixed digester contents. The photo below shows a propeller style mixer that penetrates the tank wall and is angles to encourage the tank contents to spin horizontally.

It is common to design a CSTR where the mixing system is not operated continuously. Many mixer suppliers will have their systems operate for just 30-70% of the running time. The anaerobic treatment process continues despite the fact that the contents are not actively being stirred constantly.

CSTR Tank Selection
CSTR systems have been installed using a variety of tank materials including both concrete and steel. Each style has proven to provide years of robust service. The most common CSTR configuration is a round tank. The roofs on CSTR tanks can either be fixed concrete, fixed steel, floating or a flexible membrane.

Steel tanks can be installed using either stainless steel or carbon steel. Internal coatings on the tanks include glass linings or painted epoxy coatings to prevent corrosion. Steel tanks incorporate panels that are either welded or bolted together. A typical steel digester tank with cladded rigid insulation attached to the exterior is shown below:

Great care should be taken to be sure that aggressive tank contents do not harm tank internals. Corrosive conditions are common inside digesters especially in the gas/ water head space intersection. Grit laden digester contents can abrade digester tanks and equipment. Any sealant, gasket or fastener used in tank construction needs to be properly evaluated to ensure long service life.

Concrete tanks can be implemented by either poured-in-place methods or by use of precast and prestressed panels. The same methods can be used on concrete tank roofs. Precast panels require proper care be taken to join the panels in a way that ensures gas tightness.Both concrete and steel tanks can be insulated for use colder climates from a wide variety of insulation types including spray-on or rigid insulation board.
Tanks are found that are both short and wide or taller and narrower in design. Either configuration can work well. For any given application there will be an optimum design that provides the best cost and the best functionality to maximize gas yield and minimize service and maintenance.

Digester Tank Covers
There are several styles of cover for a wet digester tank. Covers can be either fixed or floating. They can be made of a rigid material, like steel or concrete, or they can be made of a flexible material such as a canvas membrane. Flexible membrane covers generally have an inner cover that is able to flex as gas pressure changes and an outer membrane that is kept fully inflated by a small fan such as in the photograph below.

Tanks that have a rooftop mixer will require a rigid roof structure to support the mixer. See photo below for a typical digester steel roof using bolted stainless steel panels:

Digesters can also be implemented as earthen, lined lagoons. Lagoons have the benefit of being very inexpensive to install. They are more commonly applied in warmer climates as systems in colder climates require a more engineered approach to control temperature, mixing and gas collection. Lagoons are most commonly applied in animal manure projects or in wastewater treatment where waste volumes are large and land is plentiful.

Plug Flow
Plug Flow designs incorporates feed in one end and remove contents from the other. The most common plug flow applications are for farm based animal manure locations or sites with higher solids organics. As in all digester applications, good designs incorporate adequate mixing, material conveyance, gas removal, inert solids control and heating.

Higher Solids
Plug flow wet digesters can be configured in both horizontal and vertical designs. In a horizontal system contents can either flow through by gravity or can be pushed from one end to the other by a series of paddles or hydraulic action. Various suppliers often have their own innovative means by which the digester is heated and often there is a return stream of digestate to keep a robust population of microorganisms flourishing.

Wet Technology

Dry Digestion

Dry digesters are a batch operated style of digester used on wastes that are considered to be “stackable”. Ideal wastes will commonly contain 15-30% solids which implies that they can be made into a waste pile and are not liquid enough to store in tanks. Suitable wastes are ones that historically have been composted such as food wastes from consumers or institutions. Dry digestion can be a good means of generating renewable, biogas energy as pretreatment to a composting system for the remaining solids. Dry AD units are considered as a “High Solids Anaerobic Digestion” Technology or HSAD.

Dry digestion is accomplished in a sealed chamber that resembles a garage in its outward appearance. The waste is stacked inside the digester chamber and piled 10-15 ft high by the use of either a front end loader or an automatic conveyor. Unlike with Wet AD systems, the wastes are not ground, chopped or macerated before introduction into the dry digester. The contents are seeded and wetted with bacteria through recirculated water known as percolate. Digester contents are heated either by introducing heat through pipes embedded the garage walls or floor or through heat added to the percolate. Digester heat is commonly obtained by capturing waste heat from the onsite combined heat and power device, such as a reciprocating engine. Optimum digester temperature is in the mesophillic zone (95 F approx). In theory dry digesters optimally run with a closed loop percolate system where there is not appreciable water to be added nor wastewater needed to be disposed of. Percolate is collected at the base of the waste pile and returned to a storage tank and subsequently sprayed back onto the waste pile as a part of the closed loop recirculation process. Care must be taken to ensure the percolate spray nozzles are kept clean.

The waste piles need to have proper bulk so pile structure can be maintained while percolate is sprayed over their contents. Percolate should seep uniformly through the whole stack. Systems are designed to keep the contents fully wetted and having minimal dry spots in the waste pile. Typical void space in the waste pile is about 30% in order to have percolate properly permeate and distribute into the waste piles. To get proper digestibility of the waste pile some suppliers will advocate reintroducing a portion of digested material from a previous batch into the fresh waste pile to provide additional innoculum to stimulate digestion. Material such as yard waste is often added to the waste pile to provided needed structure. Sometimes undigested structure material can be returned back into the digester after post screening for future batches.

One technique deployed to increase gas generation and minimize digester volume is to split digestion treatment into two separate phases. The first digester accomplishes general breakdown of the waste known as hydrolysis. Here large particles are broken down into smaller ones and long molecules start to break down to smaller molecules. Subsequent methane generation is accomplished in either a separate mixed tank digester (see CSTR section) or in a tower style, upflow or downflow high rate digester. The theory here is to solubilize organics from the solid phase into liquid phase in the dry digester and their conversion into methane in a digester where the loading rates per ft3 of digester are much higher than the dry digester. This split phase approach can reduce overall project digester volume.

Material that has been anaerobically digested needs to be further composted and cured before use as a composted product. Digested material will often be sent through screening and particle size reduction after digestion in order to remove further packaging and impurities.

Common residence times in the dry digester is 2-4 weeks. Installed systems require several different garage bays in order to be able to have some bays sealed and making biogas while other cells are open and able to take in incoming wastes. There is a tradeoff between leaving organics in the dry digester longer or moving them out of the treatment chamber onto aerobic composting. The ultimate fate of incoming carbon in feedstock shows the organic carbonaceous fraction of the waste can either be converted into biogas methane or kept it in the composted digestate. One strategy to ensure proper digestate sanitation is to be sure the digestion process leaves enough volatile carbon in the waste so as to be sure and get a good, hot subsequent aerobic compost process going so it can achieve ample heat to get aggressive pathogen kill, if needed, during the aerobic compost phase in order to meet appropriate standards.

Elements of dry digester operation are automatically controlled by a microprocessor. Operational indicators and control can include:
– Percolate flow
– Biogas suction fan speed
– Digester temperature and heating
– Methane concentration
– Digester door status

Materials of Construction
The digester bays are generally long and rectangular shaped, just as a garage. The height and width of the bays are usually 8-15 feet wide and high. The length of the bays can extend from 20- 50 feet long. The walls of the digesters are made of specially prepared concrete that can withstand the elevated sulfide levels in the biogas. Concrete walls can be coated with specialty, rugged coatings, if desired however one needs to be mindful of not damaging these coatings during material movement. The doors that are used to seal in the digester contents need to be able to be firmly attached and contain the elevated operating pressure inside the bays. These doors are made from metal and are either hinged from their top or slide into place horizontally and are gas and liquid tight.

Feedstock Preparation
There are many ways that organics can be prepared for digestion in a dry digester. Some solid wastes management facilities use MRF systems to remove recyclables ahead of the digesters. Others use separated organics as their feedstock where generators have separated out the organic fraction at its source. Regardless of the source there are inevitably some impurities that are mixed in with the organic feedstock. Impurities can be removed either pre-digester, pre-composting or post composting. Traditional contaminant removal systems such as screens, magnets, shredders or air blow systems can all be effective depending on the quantity and nature of contamination.

Dry digesters are fed from a common entrance side. This allows for wastes to be managed in a building where potential waste pile odors can be more easily controlled. Building HVAC systems are often designed with 5-10 air changes per hour and the vent air is scrubbed by a biofilter or caustic scrubber.

Gas Management
The biogas generated during the process will be used in ways similar to other digester styles such as on-site electricity generation, gas grid injection or vehicle fuels. The biogas will sometimes need to be scrubbed for sulfides, CO2 and / or water vapor before use. The gas will often be stored in a tank with a flexible cover or in a flexible plastic receiver in the upper attic area of the process building.

Dry AD units’ chief advantages are that the units minimize system energy demands by keeping the waste in a stack during digestion and the need to dispose of wastewater from the site is minimized.

Project Examples:

• University of Wisconsin, Oshkosh. 6,000 tons/year of food waste from campus, plus yard trimmings from community. BIOFerm Energy Systems.

• Fraser-Richmond Soil & Fibre, Richmond, British Columbia. 30,000 tons/year of food waste from Vancouver area, plus yard trimmings. Harvest Power, technology from Gicon.

• Zero Waste Energy Development Company (ZWED), San Jose, California. 50,0000 tons/year of food waste from City of San Jose and other regional generators, plus yard trimmings and the organic fraction remaining after processing recyclables and garbage at the GreenWaste Recovery, Inc. MRF in San Jose. ZWED, using Kompoferm technology.
Noteworthy dry digester feasibility study

Dry Digester Feasibility Study
A composting facility in the Northeast US commissioned a feasibility study to consider dry digestion of the wastes they manage such as food waste, paper mill sludge and animal manures. They utilize static pile and turned windrow, and in-vessel composting.. The study was based on processing food manufacturing by-products and yard trimmings in a 20,000 tons/year facility. Initially, wet digestion technologies were investigated. For Mass Natural, the moisture content of the digester effluent would be too high to compost without a dewatering step. Since Mass Natural does not have municipal sewer service, storage and removal of the excess moisture from the site would have been prohibitively expensive. Although land application of liquid digestate is an option, it too would have required construction of very large storage tanks, and then contracting for land application on land owned by other parties. Mass Natural also anticipated that permitting requirements for a wet AD facility would have been more onerous if millions of gallons of liquid digestate per year had to be stored and land applied. Therefore, the project focused on dry-batch digestion options.
Mass Natural Fertilizer is a nearly ideal site for an anaerobic digester for multiple reasons: Already permitted to compost organics; local electrical distribution line is capable of receiving considerable current from an on-site engine generator; existing equipment (front-end loader, rotary drum aerobic compost vessel, agitated bay compost system, deck screens, trucks, etc.) and personnel to handle large volumes of organic material; many years of experience with, and understanding of, the local organics market; and can compost the dry fraction of the digested effluent to add value to its current product.

Proposals were solicited from two German companies.
Mass Natural established a goal of generating 848 kW from approximately 19,000 tons/year of identified feedstocks. Table 1 summarizes the biogas output/ton of potential feedstocks that Mass Natural would process annually. Both technology providers supplied data on expected biogas and methane production from these various feedstocks to supply 848 kW. While there are differences in the biogas output per ton of individual feedstocks, the weighted-average output did not vary considerably.
One of the critical questions this answered was whether or not the feedstocks can produce the desired 848 kW. Based on the biogas potential and the methane content, the calculated feedstock requirement for the scenarios ranged from 40 to 49 tons/day of feedstock. The sources of potential total feedstock flow is 52 tons/day, adequate to produce the desired electrical output.
As a further check, Mass Natural’s feedstock amount and desired electrical output was compared to the combined data from reference plants from both technology providers. While Mass Natural’s goal of 848 kW derived from the suggested feedstock amount of approximately 19,000 tons/year is greater than the average electrical output per ton of feedstock, it is within reasonable performance parameters of existing European plants. It should be noted that the feedstocks identified in Table 1 were chosen specifically for their potential methane output, whereas many European plants are designed with waste disposal as a primary goal and energy production as a secondary goal.
The Jenbacher JS3 316 genset was evaluated for this study. These units are known for their electrical efficiency as well as their ability to easily capture waste heat from the intercooler, oil cooler, engine jacket cooler and exhaust stack. For Mass Natural, electrical efficiency is the most critical of these, but the site is interested in capturing thermal energy from the engine to dry materials on site. At full output, the genset has a heat rate of approximately 9,400 Btu/kWh, which translates to an electrical efficiency of 36.3 percent. Given typical operations and maintenance schedules, one can expect an engine genset to operate at full capacity a maximum of 90 percent of the hours in a year. In addition, a dry digestion system will typically use about 7.5 percent of the rated electrical output to run the pumps and controls of the plant. Therefore, Mass Natural can expect to export a maximum of 6,128,496 kWh/yr.

Operating costs were estimated by the Brendle report to be approximately $130,000/year assuming 1.5 full-time equivalent employees. Costs for the CHP operation are estimated at $137,000/year, for a total operating cost of $267,000/year. Project revenue is estimated at approximately $1.9 million in the first year of full operation. That revenue is based on an average tipping fee of $45/ton of feedstock, and receipt of $0.10/kWh of electricity generated.Assumptions for project financing were based on 35.5 percent of total costs financed with solid waste bonds, 23 percent by investment tax credit, and the balance by equity investment. With this combination of revenue and financing, the estimated return on investment, EBITDA (earnings before interest, taxes, depreciation and amortization), divided by predebt capital costs, is 13.4 percent. This yields a payback period of 7.5 years.
The Mass Natural feasibility study concluded that a high-solids anaerobic digester would be complementary to the company’s existing composting operation. Ancillary benefits would include over 3.8 MMBtu/hr of thermal energy for on-site use and over 3,250 greenhouse gas emissions credits for sale. However, for a 20,000 tons/year facility, this was determined by Mass Natural to be a marginal investment. The company intends to conduct further evaluation to identify less costly high solids dry fermentation options.

Dry Digester Technology