Technologies: Aerobic Digesters

Commercial vendors are promoting aerobic digesters to California businesses and institutions that generate large quantities of food waste, such as restaurants, hotels, grocery stores, universities, and prisons. This webpage provides information that may be helpful when evaluating whether this type of technology is appropriate for your facility. CalRecycle strongly recommends that generators contact the entities indicated below, based on the type of technology and proposed end use for material generated, for assistance in meeting local program requirements.

Evaluating the Cost and Return on Investment

When considering the purchase of such a machine, a business owner that generates food waste should calculate the initial and ongoing costs of purchasing an aerobic digestor and compare it with the cost of separated organics collection service. Although vendors are promoting aerobic digesters as cost-efficient solutions for managing organic waste on site, this may not always be the case. The true cost of the equipment includes:

  • Installation and shipping
  • Electricity needed to operate the unit
  • Ongoing general maintenance and repairs
  • Unexpected repairs due to operator error (such as putting metal utensils in the machine and damaging it)
  • Ongoing costs such as enzymes or additives
  • Potential fee increases for wastewater (sewer) service

Generators should check with their local wastewater system operator, as operating an aerobic digester may increase the cost of their sewer services. Wastewater treatment rates for commercial and industrial customers are based on the amounts of flow, biological oxygen demand (BOD), total suspended solids (TSS), and other potential constituents of the wastewater stream. Increases in those values are likely to increase the rate that commercial and industrial establishments pay for sewer service.

How Aerobic Digesters Work

  • Aerobic digesters operate with electricity and are installed in commercial settings where food waste can be separated from other waste and placed into a unit located near the food preparation area. They typically require a drain connection.
  • Aerobic digesters typically break down food waste using a shredder or grinder as a first step and additives (typically biological agents like microorganisms or enzymes) as a second step. These units may require a continuous addition of fresh/potable water, which helps clean the system and replenishes the water lost through the discharge into the wastewater sewer system.
  • Food waste material is broken down in an aerobic environment that may include mechanical turning or agitation of the slurry. Retention times in the digester may be a few hours or a day.
  • Most aerobic digesters need regular infusions of the additive, which is usually purchased from the manufacturer.
  • Food waste that has been partially digested is not graywater, which is clear in color, low in turbidity, and comes from the drainage of bathtubs, showers, bathroom washbasins, clothes washing machines, and laundry tubs.
  • Solid product (digestate) that is removed from some types of digesters is not compost. Compost is a stable product that is the result of a complex chemical-biological degradation process. Composting takes weeks or months, not hours or days.
  • Removing food waste from the waste stream will lead to reduced hauling and disposal costs. These may be balanced by additional costs for wastewater (sewer) service.


First, a business owner should consider whether an aerobic digester can be placed in proximity to people handling food waste. Second, a business owner should also consider how to manage airborne emissions, liquid emissions, and solid output.

Wastewater Considerations

Local and regional wastewater treatment authorities treat wastewater so that its discharge meets standards established by the Regional Water Quality Control Board. For industrial or commercial establishments, pre-treatment programs impose limitations on what may be sent down the sewer lines, in order to ensure there is no negative impact on the treatment process or the authority’s ability to meet their waste discharge requirements. In addition to flow rate, biological oxygen demand (BOD), and total suspended solids (TSS), nitrogen and pathogen levels in the effluent are other factors that may be measured in the wastewater generated by each commercial customer.

If you are at a facility that treats its own wastewater, check with your water treatment staff before installing an aerobic digester.

Solid Waste Considerations

Though the volume may be significantly reduced, partially digested food waste is still considered solid waste. Not all aerobic digesters generate a solid output; many send their entire output down the drain. If the unit under consideration does generate a solid output, sometimes called digestate, then the following considerations should be made.

Solid Waste Removal Considerations

State regulations require the owner of any business remove solid waste at least once a week.

Solid Waste Handling and Transportation Requirements

Local governments license solid waste self-haulers. If a generator intends to self-haul the digestate, check with your local (city or county) government to determine how to comply with local rules regarding solid waste handling and transport. If you are hiring someone other than your solid waste hauler to remove that material, check their local government licensing status.

Environmental Health Standards for Compost

As previously stated, digestate is not compost. However, digestate could be feedstock at a composting facility permitted to accept this material type. Digestate should be blended with other compostable materials, such as greenwaste, and then be composted. Finished compost produced at a permitted composting facility must meet CalRecycle’s environmental health standards before it can be sold.

Land Application Requirements

In some cases, digestate may be a suitable fertilizer. However, fertilizer manufacturers must be licensed and registered with the state if they sell their products and make claims about the nutrient value of the end product or its suitability for use in organic food production. The sale of fertilizers by unlicensed manufacturers is against the law. Fertilizer manufacturers are inspected annually. The California Department of Food and Agriculture’s fertilizer program oversees fertilizer licensing and registration in California.

If the digestate is to be directly applied to land, with no claims about its nutrient value or suitability for organic production, then it must meet CalRecycle regulations for land application of compostable materials, which include digestate (see CCR 14, Section 17852 (a)(24.5). If compostable material that fails to meet these standards is applied to land, the material is considered disposed. The disposal of compostable material requires a permit. Disposal without a permit is illegal and will result in enforcement action.

In addition to CalRecycle regulations, California Water Board regulations may also apply. Anyone considering sending digestate to direct land application should consult with their Local Enforcement Agency and their Regional Water Quality Control Board to determine what is required.


Dehydrators Liquefiers

For more information contact: Food Scrap Management,