Odor issues at Mariposa MSW composting facility


Courtesy of Engineered Compost Systems (ECS)

The Mariposa County MSW composting facility was built at the county landfill to remove some dry recyclables from the waste stream and to produce compost for daily cover. One key motivation was to increase the lifespan of the landfill and to postpone a costly long-haul. The facility was permitted, designed and built by a team of engineers and vendors. It began operation in the summer of 2006. ECS supplied the in-vessel composting technology, the mixer, the curing system, and the biofilter and associated air handling equipment. Since November of 2006 the facility has been receiving odor complaints from neighbors.

Facility Description

The facility is designed to receive about 60 tons of MSW per day on its tip floor. The material passes through a manual and mechanical preprocessing system where cardboard, large objects, ferrous materials, and a considerable amount of plastic are removed. The remaining materials, which are approximately 60% of the tip weight, are conveyed to the compost hall and deposited into a vertical mixer. The tip floor and pre-processing equipment are located in an openly ventilated building.

Water is added in the mixer which also affects some size reduction. The material is discharged into a pile and loaded into the adjacent compost vessels with a front-end loader. The material generally stays in the vessels 20+ days where Oxygen and temperature levels are kept within target ranges. The vessels are sealed. The compost hall contains the eight composting vessels and is maintained under negative pressure. The composting hall exhaust air is humidified and scrubbed in a biofilter. The process exhaust air from the vessels is also scrubbed in the biofilter.

After composting in the vessels, the material is transferred by front-end loader to an aerated curing floor that is under a roof but open on three sides. The curing area uses a negatively aerated static pile (ASP); the process air is pulled through the piles into an in-floor aeration system and again exhausted to the biofilter. After another 25+ days the material is taken from the static piles and passed through a trommel screen to remove oversized items (mostly plastic). The product is then taken to the land fill for use as cover material.

Odor Issues

During the spring and early summer of 2006 record rains overwhelmed the water re-use storage tank at the facility - it had to be pumped and hauled numerous times. One of the sources of water was the biofilter, both by direct precipitation and run-on from the large paved areas around it. To reduce the amount of water entering the plant, a roof was retrofitted over the biofilter. During this construction the profile of the biofilter media was changed. At the same time the exhaust air humidifier was turned off since it also caused more water to collect under the biofilter. Over the summer and into the fall, the media dried out. This, plus the change in profile provided major short-circuit paths; the media was no-longer doing its job.

In the fall of 2006 nearby neighbors (as close as 400 ft from the curing area) began registering odor complaints. ECS was notified of these odor issues in December of 2006, and came on site in early January of 2007. The problem with the biofilter was immediately diagnosed and rectified within days. The media was rewetted and re-formed, additional media was added, and the humidifier was put back into service. Unfortunately by that time a number of neighbors had already been strongly impacted and were quite upset.

Once the biofilter was functioning per design, the odor emissions were significantly reduced both in frequency and severity. The compost related odors that have been reported since are the result of outdoor activities (there are other odors are associated with the tip floor and the landfill). The activities associated with odors have been identified as: 1) Transferring the material from the vessels to the outdoor curing piles; 2) Surface odor emanating from the curing piles: and 3) Screening the material coming out of the curing piles. The fundamental challenge is that composted MSW tends to form little agglomerates (0.2-2.0 cm) when wetted and mixed (see photo). Their principle constituents appear to be paper that is pulped by the mixer and film plastic. These agglomerates are relatively impermeable until they are mechanically broken up - which occurs when moving material with a front end loader and during screening device.

Remediative Steps

Adding AC Composter covers to the curing piles has stopped the fugitive emissions from the surfaces of the piles. But the ASP pile building and final product screening activities are still releasing odors. The approach we have recommended is first to break up the agglomerates so that more effective composting can be achieved in the vessels. The following recommendations have been made to achieve this:

  • Add less water to the initial mix
  • Pick more plastic during the sorting process
  • Remove and remix the material mid-way through the in-vessel composting cycle
  • Consider adding a bulking agent
  • Improve the quality of the feedstock coming in through better diversion programs and education.

The operators are working on all of these fronts and have made progress over the past few months. Since it takes approximately 50 days for material to work its way from tip floor to final screening, improvements made on day 1 don't show benefits for quite some time. One of the other challenges managers have had is finding, hiring, and training staff while starting up a new facility. Another challenge has been the waste stream itself, which has significantly more film plastic than was indicated in the waste audit from the late 1990's.

If the remediation steps take longer than is politically palatable to address the problems, the other solution is enclosing the curing and screening area, and treating like the composting hall. It may come to this.

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