Mariposa compost facility odor remediation update


Courtesy of Engineered Compost Systems (ECS)

Staring in the winter of 2006 the staff at the Mariposa Compost Facility worked quickly to make process changes to bring odor issues under control. Here is the latest chapter in their story.

Odor (incident) Logs

In January 2007 the Local Environmental Agency (LEA) provided 'odor logs' to the county residents that were troubled by the compost facility's odors. The odor logs were collected regularly and the recorded data (time, date, type of odor, etc) along with weather conditions were input into spreadsheets. These spreadsheets became the empirical measure of success for the process changes implemented at the facility.

Process Changes

ECS engineers visited the site in January 2007 and made the following process recommendations:

  1. Lower the moisture content in the initial raw mix;
  2. Remix the compost after an initial period of primary composting; and,
  3. Increase the vessel aeration rates prior to vessel unloading.
  4. Remove as much film and other plastics from the raw compost as possible;
  5. Obtain wood chips for use as bulking agents; and,
  6. Use AC Composter&tm; covers to contain odors during secondary composting

1. MSW feedstocks are typically rich in paper fiber. Lowering the moisture level was intended to reduce the forming of paper-wads (agglomerates). The agglomerates formed anoxic pockets that released odors when broken apart during loading and unloading the vessels. Lowering the raw compost moisture levels to approximately 60% did reduce the forming of agglomerates.

2. After several days in primary composting the degradable materials soften. Removing the compost from the vessels and running it through the heavy duty compost mixer breaks open the agglomerates that form even with reduced moisture. Remixing also exposes more degradable surfaces and further homogenizes the heterogeneous MSW mix.

The benefits realized by implementing steps #1&2 included decreased the time to come up to PFRP temperature; and reduced odors. This was proven by decreasing entries in the odor logs.

3. The operators now manually select a high airflow setting on the ECS CompTroller&tm; aeration control software 24 to 48 hours prior to removing the compost from the vessel for any purpose (either to remix or move to secondary composting). This effectively saturates the compost with Oxygen, lowers the compost temperature, and scrubs compost odors from the biomass. Previously there was a clear association with removing compost from the vessels and the number of odor incidents recorded during loading and unloading. The reduction in odor incidents logged shows this high aeration technique is successful.

4. A huge amount of film plastic found in the Mariposa MSW. They have altered their pre-processing to manually pick more out by hand prior to the mechanical sorting process. It took the facility a while to fill the sorting positions (allocate funding and find staff) to bring the staff up to a full working compliment. Sorting staff are now directed to remove as much plastic as possible. Reducing the amount of film plastic improves the air flow through the feedstock, which we believe will further reduce odor generation.

Reducing the amount of plastic in the waste stream that reaches the Mariposa facility is an uphill battle. It will take years of infrastructure development (collection programs and opportunities to recycle) and education to significantly reduce the amount of plastic entering the facility. The aggressive sorting will need to continue until new programs are in place.

5. Last winter was wet and included a lot of snow. The loads of MSW that were placed on the tipping floor sloshed with free liquid and melting snow. Even without adding liquid from the storage tanks (to reduce moisture levels in the initial mix) the raw compost was just too wet. Staff has now found sources of wood waste and are stockpiling and drying it at the compost facility. The ground wood will be used to bulk the incoming feedstocks when the winter rain and snow again increase the moisture to unacceptable levels.

6. The original compost facility plan called for enclosing the Aerated Static Pile (ASP) area for secondary composting, however, in an effort to save capital costs, the building walls were omitted from the contractors scope of work. Since the time the facility was built, ECS has developed the AC Composter&tm;, an ASP system using pile covers, negative aeration, and biofilter to contain and scrub odors. The AC Covers were added to the negative aeration system that was already in place and the combination has successfully controlled the odors associated with secondary composting. The same high aeration setting described in #3 above is used prior to removing the covers and moving the compost to the screening area.

Since these process changes have been made the number of odor incidents logged in the county odor logs have dropped dramatically. (From 18 complaint-days in March, to 3 complaint-days in June.) During this process the staff of the Mariposa County Compost facility has learned many valuable lessons. These lessons, and further updates of the continual improvements at Mariposa, will be the topic of a future Blog.

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