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Omak Biosolids Composting Facility
Omak
4,545
Theadore W. Pooler
Project Engineer
509-966-7000
City of Omak
Biosolids Composting Facility
May 13, 2002
Omak implemented a Biosolids composting program to provide beneficial reuse of the sludge
(biosolids) produced at our wastewater treatment plant and used a solid waste product that would
otherwise go to the landfill. We tell our success story below.
What it is-Why it was Needed
Biosolids are a by-product of the wastewater treatment process. The Washington Department of
Ecology encourages beneficial reuse of biosolids, and developed regulations and standards to
protect the public and the environment when biosolids are reused. Unfortunately, our older
method of handling biosolids did not meet the new State or Federal regulations. We applied
untreated liquid biosolids to our 200-acre land application site. Little pathogen treatment
occurred and there was no direct beneficial use of the material.
Handling the liquid biosolids was also difficult and expensive. We pumped sludge into a tanker
truck and hauled it to the land application site, so we were hauling large volumes of water to
dispose of the small quantitiy of biosolids produced by the wastewater treatment process. Land
application was impossible during the winter, or when the ground was too wet for spreading with
the tanker truck. Therefore, we had to keep excess solids in the treatment process, limiting our
treatment plant capacity and pushing the limits of our Ecology permit to discharge to the
Okanogan River.
Omak had to find a cost-effective means for treating and disposing of our biosolids. Our
engineer, Huibregtse, Louman Associates, Inc. looked at options. They considered treatment to
meet Federal standards for Class B biosolids, which are suitable for application on agricultural
land, provided site access is restricted and certain food crops are not grown. They also
considered treatment to meet standards for Class A biosolids, which have a higher level of
pathogen removal and can be distributed to the public. After weighing the options and costs, and
accounting for the increasing concerns about the use of Class B biosolids, the City council
decided to move forward with biosolids composting. Though more expensive than the Class B
options, we went with a biosolids treatment process that provided both a higher level of
treatment and the greatest potential for beneficial reuse.
We use the Containerized Compost System™, developed by Engineered Compost Systems, of
Seattle Washington, to treat our biosolids. Dewatered sludge, known as biosolids, is mixed with
amendment (wood chips, sawdust, and ground yard waste) and placed into 40 cubic yard
containers, where it is treated under controlled conditions to produce composted Class A
biosolids suitabel for public use. The Federal regulations (40 CFR 53) and State regulations
include invessel composting as an approved process to further reduce pathogens in biosolids.
How it Operates
The Omak wastewater treatment process produces waste acitivated sludge. This necessary by
product is 1.5% biological solids (biosolids) and 98.5% water. For use in composting, most of
the water must be removed. The waste activated sludge is pumped to a belt filter press that
removes 90% of the water, and the water is returned to the treatment plant for further treatment.
We then conveyed the biosolids to our 325 cubic foot mechanical auger mixer where we
combined them with amendment according to special “recipes” developed by our treatment plant
operators. These “recipes” optimize the moisture content and particle size necessary for the
airflow and bacterial growth needed for successful composting. Batches of the mixture are
placed in the compost containers for 20 days of processing. We screen the cured compost and the
final product is a high-quality soil additive. Key features of our composting process are:

In-vessel composting, using the Containerized Compost System™ provides an
enclosed treatment system. Enclosed systems control odors, improve process control,
provide product consistency, and operate in cold weather.

We have a readily available, low cost supply of amendment. The local lumber mill has
excess wood chips and sawdust, which would otherwise go to waste. Omak also obtains
free wood chips and ground yard waste from area landscapers and City maintenance
crews.

The Containerized Compost System™ uses computerized process monitoring to control
the composting. Probes inserted into the containers monitor temperatures, and airflow
through the containers is automatically adjusted to maintain optimum composting
conditions. The City is assured regulatory compliance since the system records the
temperatures to document that the process meets treatment standards.

We have six 40 cubic yard containers that local roll-off trucks, used to transport the large
garbage containers, can move. We contract with a local hauler for occasional trips to the
site, and did not have to buy our own truck. Two more containers can be added in the
future.

Compost is kept in the containers for about 20 days. Heat from the composting kills the
pathogens, and decomposition breaks down the biosolids and amendment into a useable
material.

Excess heat from active containers warms other containers, helping with start-up. Waste
air passes through a simple biofilter, containing treated compset, to remove odors.

After the 20 days of treatment, we screen the compost. Fine, useable material passes
throught the screen and is collected for the final use. The coarse material is discharged
separately and reused as part of the mixing “recipe”.
What it costs
General Industries, Inc.of Spokane, constructed the improvements for $595,000. This price
includes the compost mixing building, the compost control building, concrete slabs, and the
compost equipment. We found the compost equipment is sales tax exempt per WAC 458-2013601, since the City purchased the equipment for direct use in a manufacturing operation.
We incur annual cost for electric power and one person, who also shares time with other
treatment plant operations. Estimated annual operating costs are $35,000. We can sell the
compost, but are still developing the market. Annual revenue from compost sales will not offset
the operating costs, but it should provide the City $8,000 to $12,000 per year.
How it Benefits the Community
Biosolids composting meets the Department of Ecology’s goal for beneficial reuse of biosolids.
We chose to go the extra step to meet the Class A treatment standards because it opens other
opportunities for beneficial use. Composting benefits our community in the following ways:

We provide a low-cost biosolids product (compost) for community use. The
Class A biosolids have a low pathogen content and are suitable for direct distribution to
the public. The City actively demonstrated the visability of the material, using it in City
landscape areas and parks. Commercial landscapers use the material extensively for a
variety of their soil ammendment needs, including lawn preparation and flowerbeds. The
public is using the materials in gardens and flowerbeds. We can put 300,000 pounds (dry
weight) of biosolids to beneficial use, rather than putting untreated solids on a land
application site with no real benefit.

Compost improves Eastern Washington soils. By increasing the organic content of the
soil, our compost aids in moisture retention and improves workability. Though compost
has a low nitrogen content, it does provide a slow-release form of nitrogen to augment
plant fertilizer needs.

Compost is sold to the public to generate revenue for the sewer fund. Frequently,
communities incur costs for disposal of Class B biosolids.

Our composting process uses a wasted wood product (chips and sawdust) that is of no use
to the lumber industry. Though most of the wood chips and sawdust generated at the mill
are put to use, a portion must be wasted. We use that waste product.

We also use ground yard waste and put it to beneficial use, rather than sending it to the
landfill. By using these combined sources of amendment, we divert about 2,100 cubic
yards of material away from the landfill waste stream.