Commercial vendors are promoting food waste liquefiers, also referred to as “biodigesters” or “wet” systems, to California businesses and institutions that generate large amounts of food waste, such as restaurants, hotels, grocery stores, universities, and prisons. This webpage provides information that may be helpful when evaluating whether or not 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.
- Food waste liquefiers operate with electricity, require a drain connection into the waste water sewer system, and are intended for installation in commercial or industrial settings.
- Food waste must be separated from other wastes before being placed into this equipment, which is typically placed near the food preparation area.
- Food waste liquefiers typically break down food waste using a shredder or grinder as a first step, and a secondary step may use biological agents (e.g. microorganisms and/or enzymes) as additives and require a continuous addition of fresh/potable water. The fresh water 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.
- The residual food waste slurry is discharged into the wastewater (sewer) system.
- Food waste that has been liquefied is not graywater, which is typically clear in color, low in turbidity, and comes from the drainage of bathtubs, showers, bathroom washbasins, clothes washing machines, and laundry tubs.
Residual Liquids From Wet Systems
The residual effluent from food waste liquefying systems should not be utilized directly for irrigation. Food waste liquefier systems should be connected directly to the sewer system so that the effluent can undergo further treatment at the waste water treatment facility.
- The residual material from a food waste liquefier will contain a relatively high level of biochemical oxygen demand (BOD). This is a measure of the amount of oxygen that would be required to finish breaking down the organic material left in the residual liquid. Any entity considering installation of a liquefier should check with their local or regional waste water treatment authority to assess the authority’s operational ability to treat the waste. A high BOD value may pose a problem for some wastewater treatment systems.
- 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 discharge requirements. Such programs typically require measurement of flow levels, BOD, and total suspended solids (TSS). Nitrogen and pathogen levels in the effluent are other factors which may be measured in the wastewater generated by each commercial customer.
- Wastewater treatment rates for commercial and industrial customers are based on the amounts of flow, BOD, 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.
- If you are at a facility that treats its own wastewater, check with your water treatment staff before installing a liquefier.
Proper Handling of Food Waste
Food waste is considered solid waste in state regulations, and is required to be removed from businesses and residences at least once each week and taken to a facility designed and permitted to handle such waste. In addition to state requirements, solid waste handling may also be subject to local ordinances.
Incorporating liquefied food waste into soil is disposal. Only operations that meet all local and state requirements can legally dispose of solid waste, including liquefied food.
Liquefied food waste can be a feedstock for an anaerobic digester. Check with your local recycling coordinator to find an anaerobic digestion facility handling food waste near you. Hauling costs for liquid waste should be considered in any calculations regarding a liquefier where wastes will not be going down the drain.
Liquefied food waste could possibly be a feedstock for composting. Food waste that has been dried or liquefied is not compost. Check with your local recycling coordinator to find a compost facility near you which is permitted to accept food waste.
Food waste may potentially have value as an ingredient for animal feed or fertilizer. Liquefied food waste would need further processing to become fertilizer. The California Department of Food and Agriculture regulates the manufacture and distribution of effective and safe animal feed and fertilizer.
Literature indicates that there are potential problems with sewer line clogs or “slugs” associated with liquefied food waste. These can be difficult to detect until the clog is quite large, as well as unpleasant to clear. Please check with your sewage system operator before purchasing a liquefier. When a food waste reduction strategy includes liquefiers and the use of public sewage lines and public wastewater treatment plants, the entities in charge of the sewage line and wastewater treatment facility need to be notified, additional sewer fees may be assessed, and the facilities utilizing a liquefier would need confirmation that the treatment system will recycle the liquefied food, as the end use for the material will factor into the determination if this system meets local food waste diversion program requirements.
From a climate change perspective, diverting food waste from landfills will likely achieve substantial greenhouse gas (GHG) emission reductions due to avoided methane emissions at landfills. Landfill emission reductions will likely overshadow other reductions or emissions that are associated with any organics management option chosen. Depending on how the food waste is handled, there could be additional GHG emission reductions (e.g., biofuels/bioenergy from an anaerobic digestion facility or a digester at a publicly operated treatment works [POTW]). There could also be additional energy burdens which would translate to GHG emissions (not reductions), e.g., liquefiers require energy and water inputs and consume front-end capacity at POTWs that require additional energy inputs (rather than putting food waste directly into a digester at a POTW). Even when food waste is liquefied and sent to a POTW that ultimately returns biosolids to a landfill, there are some GHG emission reductions and tonnage diversion from landfills; however, at diminished levels due to considerations mentioned above.
The incremental energy demand to move the additional food-laden feedstock through the front end of the treatment works can exceed the incremental extra energy produced in the digester from the food; and therefore, there may not be a GHG incentive to put
foodwaste into the front-end of a POTW (which is what happens when it goes through the sewer system). Some of the potential energy value of the food can also be lost as it moves through the sewer lines to the POTW. There could potentially be some
isolated circumstances where liquefying food waste and sending it through the sewers to a nearby POTW would be a reasonable climate alternative if this option has been fully vetted between the generator and the POTW--with acknowledgement of the increased
load on the POTW. This scenario most likely would be the exception given the points above and other infrastructure considerations, e.g., sewer lines are typically not designed to handle large volumes of food.
Few studies have been performed on this kind of technology. One study conducted at Loyola Marymount University in 2012 concluded the following:
- Effluent is more concentrated than raw sewage typically entering a sewage treatment plant.
- Effluent contains high levels of fecal indicator bacteria (mainly total coliforms and enterococci).
- Effluent contains a high nutrient value. However, bacterial data indicates potential human health concerns.
For More Information
- BioCycle Magazine. "Analysis of Biodigesters and Dehydrators to Manage Organics On-Site," by Zoë Neale.
- The Composting Collaborative Pretreatment Directory for information on wet and dry Food Waste pretreatment systems.
- Food Waste Dehydrators