A slate of new legislation in California has spurred a movement to remove all food waste from landfills, and the Sanitation Districts of Los Angeles County (LACSD) has found a win-win solution via digester addition.

By injecting processed food waste into their existing sludge digesters, they remove a significant portion of the region’s food waste stream while also potentially doubling their gas production.

“We started a demonstration project with Waste Management years ago — it was a full-scale project to figure out if we can actually co-digest food waste slurry in our anaerobic digesters at the wastewater treatment plant,” says William Chen, supervising engineer over energy recovery. “There is a big call and move to divert organics, specifically food waste, out of the landfills.”

The LACSD service area generates about 4,000 tons of organic waste per day. The projected maximum that their treatment plant could help manage is about 550, including both digester capacity and accepting food waste in their wastewater stream.

“At this time, the legislation really doesn’t have any teeth yet, there’s no financial ramifications or penalties, so we don’t have the 4,000 tons knocking on our door right now. As an interim, we are actually diverting all of our food waste to the headworks of the facility,” Chen says. “There’s capacity at the headworks to take our food waste without really expending much capital at all, we just treat it as incoming sewage. And that’s going to buy us some time to see where the market goes.”

By treating the food waste as sewage instead of injecting it directly into the digesters, they lose about 60% of the production potential. So while it doesn’t require additional capital, it’s less efficient and they are losing out on theoretical net gain.

Also, the persisting drought conditions in recent years have left extra digesting capacity in the system that would otherwise go underutilized.

“It’s gotta go somewhere, and there’s really not too many places for it to go. Either composting, wastewater treatment plants or anaerobic digesters,” Chen says. “We had to first off make sure that that was even possible.”

The pilot project

LACSD has 24 digesters in its treatment facility. Right now they are designing a receiving station for processed food waste that would feed into five of those. Their test project added food waste slurry to two of them as a proof of concept.

“It’s pretty much a slurry that we would end up taking at our wastewater treatment plant and slowly meter it into our anaerobic digesters on site,” Chen says. “We dedicated two of them, and closely monitored everything — a whole gamut of constituents that we were looking at to make sure the biological process was not affected, because obviously wastewater treatment plant operation was paramount, we didn’t want to affect that.”

They found that not only was the digester injection an effective way to dispose of organic waste without disrupting normal wastewater treatment operations, it also had a much greater impact on gas production than anticipated.

“We did that for about 2.5 years. The maximum capacity for each digester was 62 tons of diverted food waste per day,” Chen says. “We were also seeing a close to 100% increase in digester gas produced from that digester.”

After the success of the project, LACSD’s in-house design department began making plans for a receiving station and food waste injection to five digesters.

“The eventual plan is to be able to operate that station and feed it into five digesters. We would be able to handle and accept up to 310 tons per day of food waste,” Chen says.

During the pilot project, they were adding 10% food waste to a mixture that was 65 to 45 percentage mix of primary and secondary sludge. Additional mixing energy was sometimes needed depending on the solids content of the food waste sludge, which could be up to 14% solids. 

Ahead of the curve

Though it’s not a new idea, most other utilities are still just talking about it.

“Everyone’s kind of in a holding pattern right now, and trying to feel out what the market’s going to do, just because it is a huge undertaking,” Chen says. “We’re thinking in the next 2 or 3 years, so right now we’re staying steady. We’re able to take food waste at the headworks of the plant, and obviously that’s kind of the easy button. We have capacity for up to 350, so once we surpass that, and once we lock in more agreements, then we can kind of say, OK, there really is a need for a receiving station, we’re going to invest on this energy recovery project. The receiving station that we are planning to build, obviously, is going to cost quite a bit, and so we’re not going to really invest that capital until we see there’s a need for that.”

If the market demand increases enough, Chen said they could feasibly consider adding food waste to all 24 digesters, but there are a lot of factors to consider.

The plant utilizes digester gas in its self-contained power grid, and excess gas will be diverted to conversion system for recycled natural gas.

“We’re building a new biogas conditioning system, which is going to take some of the anaerobic digester gas, and we’re going to clean it up through membrane processing. That’s going to tie in directly to an on-site CNG fueling station,” Chen says. “Once we get the gas, we’ll be able to sell it, which is how we would justify the up-front capital investment — because ultimately, it is for the increased efficiency in gas recovery.”

There is additional infrastructure that must be designed and built before the receiving station — the conditioning systems and pipelines to carry the refined gas, for example. They have decided to move forward with the infrastructure development, which they expect to be completed by June 2020.

“We’re market-driven, so for each ton that we take, it’s by the market,” Chen says. “So for us to build the receiving station, pay and cover the costs of handling biosolids, that really has to justify itself on the back end, and how these projects get funded is from the energy side.”

Not compromising treatment

“Obviously, plant operation is No. 1, so where we decided to build this, the bank of the five digesters, they’re isolatable, in case there was a big contamination, or this bank of digesters just go down for whatever reason, it wouldn’t impact the rest of the digesters,” Chen says. “There were a lot of thoughts and considerations taken to make sure that these five are isolatable, and it wouldn’t really impact anything else.”

The LACSD team is cautiously optimistic, hopeful that it can help lead the charge to fulfill the new regulations in a way that benefits both the region and their utility.

“It’s definitely an exciting project, and from our demonstration project that we did, I don’t foresee any negative impacts, so I think it’s a great idea,” Chen says. “It’s just that it’s a fairly new market. The equipment out there to preprocess and sort the material is always interesting to see, so we’re definitely excited to see where it goes, but obviously we have our reservations too, in seeing where our market goes.”