Windrow Composting – Blossom Valley

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In the middle of farms and fields, south of Bakersfield CA, in the small town of Lamont CA is one of the largest composting facilities in the United States. Driving from downtown LA, it took me 2.5 hours to make it out to the facility. Just like the surrounding farms, this facility has rows and rows of crops – except this crop is compost. Specifically, windrow composting – where heaps of organic waste, organized into rows, sits for 4 months as it slowly turns into compost. Windrow composting involves a few major steps: Collection, Feedstock Preparation, Windrow Processing and, Screening.

Collection: Before material even makes it to Lamont, it must be collected. It sounds simple but complexities exist. Material originates not only from the bin placed out at the curb in residential neighborhoods but from several different sources and even that bin can be complex. Some residential bins collect yard waste only while some collect yard waste & food scraps. The composition or profile of the waste varies differently when food scraps are added and changes depending on neighborhood. Additionally, most of the waste comes from commercial and farm sources – again with its own profile. All these sources arrive at the Lamont facility via different pathways. Residential and some commercial material is received at Recology’s Sun Valley MRF. Other commercial and farm wastes arrive directly at the Lamont facility from various sources (other MRFs, farmers truck it in themselves, etc.)

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Early morning collection in East Los Angeles

Feedstock Preparation: Successful composting starts with the Carbon to Nitrogen (C:N ratio) of the feedstock. Yard trimmings have a high ratio and food waste has a low ratio. Mixing brings the ratio within the ideal C:N ratio of 20-40:1. The Sun Valley MRF mixes the collected feedstock to be composted at Lamont.

Windrow Processing: The Lamont facility is the largest permitted composting facility west of the Mississippi. Every day, 150 tractor-trailers unload tons of organics. The logistics behind such a large scale operation can seem harrowing but the standard in large scale composting is to form windrows – long rows of material dumped out of the back of the tractor trailer and left in the field. Composting starts where the materials lies!

windrow formation

Several truckloads make a completed windrow which can be a couple hundred feet long. Once unloaded, the material must be covered with dirt to contain any gasses. And the windrow must be watered – with an ideal moisture content between 40-60%. The Lamont facility uses gray, filtered but untreated water from the Lamont wastewater plant. In addition to adding moisture, the water serves to further reduce the C:N ratio.

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Initially, the row is a lumpy mess but the material must be turned to aerate the material. Aeration occurs every 3 days during the first 15 days, with a reduction in frequency after the first 15 days. To aerate, a giant windrow turner straddles a row and churns the material as it drives over the row. As the turner passes, the row is processed into a neat pile.

The entire composting process takes about 120 days. Throughout the process, the temperature is measured o ensure pathogens are destroyed and the compost is safe to use. Temperature is also a key insight into how the composting process is performing.

With windrow composting, the long processing time of 120 days requires large amounts of land. In vessel composting is a popular alternative in space constrained areas as it requires less space and completes the process in a shorter amount of time with greater control of environment for more precise conditions for composting.

Screening: At the end, most of the material will have processed into very small particles. But, plastic bags, nails, large pieces of wood which didn’t fully decompose will have to be removed. Material is loaded into a hopper which screens out particles larger than about 4 inches. Then the material is moved through a large rotating cylindrical tube with many holes on an incline – called a trommel screen. Properly sized material slips through the holes while oversized material continues to travel up the trommel, ending in the overs bin. The overs are sent to landfill. Prime material undergoes further screening: a magnet is passed over the material to capture ferrous metals (mostly nails left in trees) and a stream of air is passed through the material and into a vacuum to capture any plastic film material. Prime compost is sold to surround area farms.

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Screening at Blossom Valley (trommel screen is seen in bottom center of the equipment)

I initially thought compost was equivalent to dirt but the classification is different. Compost is a soil amendment and is able to provide nutrients and microbes which have been depleted during traditional farming techniques. It also absorbs and retains much more liquid than just soil alone. This is great for drought stricken areas like the Central Valley, where the facility is located. The tour gave me a much greater appreciation of what is involved in processing our organics. The system is much more complex than people initially assume.

South San Francisco Scavenger SMARTFERM Anaerobic Digestion Facility

I recently went on a tour of the South San Francisco Scavenger SMARTFERM anaerobic digestion facility. Built in 2015, the facility can process 11,000+ tons per year – using methane produced as energy for the facility and as fuel for the 10+ CNG collection trucks.

One of the first things noticed on the tour is the space constraints. Less than 100 yards away from the digester is a parking structure and the Genentech executive building. These constraints require the facility to have a small footprint with tight odor controls while balancing the need to process as much material as possible. The SMARTFERM technology solves this balancing act.

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The key to successful anaerobic digestion is the carbon to nitrogen ratio (C:N) of input material. At SSF, various feedstocks are mixed for this optimized ratio prior to digestion. Generally, brown or woody material has a high ratio and green material or food waste has a low ratio.

While waiting for an available digester chamber, the material is stored in the aeration bay. The bay is under negative pressure (i.e., vacuum) for odor control. Once added to a digestion chamber, a 21 day, 3-phase process begins.

Start Phase: Anaerobic digestion only occurs in a specific temperature range. To bring material into this range, a heat-generating, aerobic decomposition process is started by pumping air into the chamber allowing the material to heat itself up naturally! It’s a clever solution with lower energy requirements than heating up the material with steam or hot air.

Fermentation Phase: Once at the correct temperature, percolate with bacteria similar to a cow’s digestive tract is sprayed onto the material, starting digestion and producing biogas. The key indicators during fermentation is quality and quantity of methane produced. Unfortunately, high quality doesn’t occur during times of high quantity. To increase quality, CO2 and other components are filtered out.

Termination Phase: After ~20 days, methane production has been greatly reduced and doesn’t make sense to continue. To stop the process, percolate is no longer sprayed onto the material. Air is again added to the chamber but this time to purge the chamber of methane and create a safe/non-explosive environment to open the chamber and allow the removal of digestate.

Remaining digestate is treated in an IVC (In-Vessel Composting) tunnel to compost the material. The tunnel also removes ammonia gas produced and processes it with an acid scrubber where sulfuric acid is sprayed over the gas to precipitate out the ammonia as ammonium sulfate. Scrubbed gas then passes through a BioFilter similar to the flora on a forest floor – removing odors and harmful gases – and into the atmosphere. The compost produced is screened offsite and sold to farms!

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Overall, the facility is an example of anaerobic digestion which is financially viable while dealing with space and odor constraints.

Although not a substitute for the tour, there is a great YouTube video summarizing the process: https://www.youtube.com/watch?v=XcjAgEYoFJE