September 22, 2019

Eco-Tip: Is the Sewer a Good Place for Food Scraps?

“If a lot of people, or even if just one large generator, started dumping a lot more organic material down the sewer, we would get out of balance.”

Because food scraps are not allowed in any local curbside yard waste carts, some people put kitchen scraps down their garbage disposal, thinking this is a viable composting alternative.

Although many local wastewater treatment plants do send at least some of their biosolids – a nice word for sewage sludge – to composting facilities, using garbage disposals for food scraps can pose environmental problems.

Water required to transport food through sewers makes this form of disposal environmentally costly in dry areas like Ventura County. Also, composting facilities for biosolids are not in Ventura County, requiring trucking both to send the biosolids from local treatment plants and again to distribute the resulting compost, usually to farms growing non-food crops.

Phil Archer, lead wastewater operator at Ventura’s water reclamation facility, at a tour of the city of Ventura’s water reclamation facility last month, explained another problem with handling food scraps through garbage disposals.

“The microorganisms we use for biological treatment of our wastewater are adjusted to an expected level of organic material in the water,” he said. “If a lot of people, or even if just one large generator, started dumping a lot more organic material down the sewer, we would get out of balance. At some point, the space we have available for adding microorganisms would not be enough.”

Wastewater Utility Manager Gina Dorrington explained another challenge faced when wastewater treatment facilities must deal with excessive organic material – material capable of rotting.

“When water conservation cut our facility’s flow from 8½ million gallons per day down to 7½, some organic material got stuck in pipes and rotted,” Dorrington said. “We didn’t have enough water pushing it through. This led to problems with hydrogen sulfide toxicity buildup in the pipes.”

That was a problem until Ventura Water made operational changes in how it cleans its sewer collection system. The toxicity upset the microorganism balance at the water reclamation facility.

Here’s what businesses do

On a commercial scale, some supermarkets and food processors use industrial-size garbage disposals from companies such as Garb-el.

Since these systems put so much organic material into sewer systems, special discharge permits are usually required prior to installation.

Wastewater treatment facilities must ensure sufficient flow of water in downstream pipes and capacity to handle the material, so permits for such installations can be expensive, requiring operators to compensate wastewater treatment facilities for infrastructure improvements needed to handle the flow.

In contrast to systems that simply chew up garbage and wash it down pipes, companies such as Power Knot (powerknot.com) are promoting on-site biodigesters.

A biodigester uses microorganisms to digest food scraps, and a screen between the device and the sewer discharge pipe is intended to ensure only digested material and liquid flow through.

The company claims most of the microorganisms remaining in the digestate-infused water die due to lack of food before they arrive at the wastewater treatment plant, the remaining microorganisms pose no threat to the treatment plants, and the amount of undigested organic material able to pass through the screen without being digested is small.

Power Knot President Iain Milnes claimed, “From a load of 1,200 pounds of food waste, the organics discharged to sewer is less than the amount of organics in two human defecations.”

These systems are likely to gain increased attention in the next few years as large generators of organic waste – ranging from schools to restaurants – could face increasing pressure to comply with State laws requiring composting of their organic waste.

In 2014, Assembly Bill 1826 applied requirements that become more stringent over time.

Most recently, as of January this year, businesses generating 4 or more cubic yards of waste per week are required to recycle all their organic waste, ranging from yard clippings to food scraps, if programs for this recycling are available.

What do the experts say?

The California Department of Resources Recycling and Recovery’s local liaison to Ventura County, Cathy McDonald, worked with a team to review the claims of Power Knot and pointed out that on-site digesters could “make a business compliant with AB 1826 regulations for the food waste material that it generates” only if such a system discharges to a wastewater treatment facility sending all its biosolids to a compost facility.

Additionally, “businesses are also required to address” their other organic waste, such as yard clippings, and cities and counties are required to set up programs prioritizing food waste prevention, including recovering food for human consumption, over food waste composting.

John Minkel, Thousand Oaks’ utilities superintendent, who manages the Hill Canyon wastewater treatment facility, pointed out that Power Knot and other manufacturers of on-site treatment systems cannot fully anticipate parameters of the systems’ discharge, which depends on what users put into them.

Local limits for accepting this discharge vary in categories such as biological oxygen demand, so “some pilot testing with representative food waste inputs and coordination with your municipality’s wastewater pre-treatment program personnel” might be wise before a company can be certain they should buy an on-site digester.

For the Hill Canyon plant, Minkel concluded, “several” small digesters could probably operate “in our service area” without significantly impacting operations. However, each case would have to be considered, he said.

One similar device is currently in Hill Canyon’s service area; it “was determined not to have a significant effect and was given a permit to operate” according to Minkel.