SUBJECT: ENG – Rotary In-Vessel Composter for mortality disposal

DATE: April 10, 2002

TO: Jerry D. Walker, Acting State Conservation Engineer

FILE CODE: 210

USDA – NRCS, Temple, TX

At your suggestion, I contacted Mr. _______ of ________ in _______ Texas, to discuss the rotary composter they market. During the course of the conversation I talked to one of Mr. _____’s associates as well. From the literature on the WWW and other material crossing my desk, I have identified at least two additional companies marketing a rotary in-vessel unit. I’m not certain if they all come from the same manufacturer, but the descriptions appear to be similar. As you know, the rotary composter was researched by Dr. Don Cawthon with Texas A&M – Commerce; first for dairy manure and later for mortality disposal and other manure streams. I saw the composter demonstrated a few years ago at a meeting in Arkansas.

The rotary in-vessel composter is marketed as a stand alone unit, but from the web literature and discussions with ________ and others, it appears ancillary components are needed. Included is a mixer to macerate the carcasses and mix with litter and bulking agents, a loader to move the material from the mixer to the composter, a secondary or second stage composting area, and most likely a building of sorts to house the entire “system.” I mention this because in my mind it greatly influences the economics when comparing one composting technique to another.

My conversation with Mr. ______and his associate confirmed that the normal in-vessel composting process does not meet the temperature-duration requirements of Natural Resources Conservation Service (NRCS) Practice Standard 317 (Composting Facility). PS 317 requires a compost mix reach a minimum of 130⁰ F. and maintain it for a minimum of five days. The use of a stacking area or “static bins” for second stage or secondary composting could allow a second heat that may well result in additional time at the 130⁰ F. temperature, but this needs to be demonstrated. I agreed to send Mr. _____ a copy of PS 317 as well as our practice standard for storage facility along with a copy of this letter.

I believe Dr. Cawthon has adequately demonstrated the rotary in-vessel composter is capable of producing a good looking compost product, but I have not seen the level of pathogen reduction testing as was done a decade ago for the bin composters. While we can assume the compost once removed from the composter will re-heat and achieve required pathogen destruction; we need _______ to provide confirmation of the combined temperature – duration relationship associated with the unit and the ancillary stacking area or bins.

During the past week I talked to NRCS folks in Michigan, Arkansas, and Tennessee where you were told they offered cost share for the composter. Only Arkansas includes the practice in their cost share list, and then with a maximum payment of $3000. There are installations in both Michigan and Tennessee, but these were demonstration situations. In Michigan parts of the system were cost shared but not the rotary drum itself.

The NRCS folks in Tennessee expressed concern about the management of this type of composter. Apparently moisture control is critical and how to achieve it is not obvious to the operators – takes a lot of trial and error that results in some smelly compost on occasion. Picked this same concern up from the web sites as well. Probably a site specific operation plan will be necessary to provide the operator with the variation in mixes needed to accommodate differing sizes of mortality and varying litter characteristics. I see this as the role of the provider of the in-vessel unit.

Let me recap. The rotary in-vessel composter is described as a “stand alone” unit, but in fact appears to need ancillary components to function effectively. I found a wealth of anecdotal information to support the use of the rotary composter for mortality disposal, but limited research - quality information. We need to have well documented temperature – duration profiles demonstrating the composter and ancillary components meet the criteria of PS 317. In light of the reports from Tennessee, I believe we would also need very detailed, site specific operation and maintenance (O&M) plans for the rotary composters and their ancillary components. While some of the material could be “cookie cutter,” other parts, such as compost recipes, need to tailored to the operation. Since the composter operates with a rotary drum, maintenance of the unit may be limited but critical to the operation. In my view the equipment supplier should provide the O&M plans, particularly while there is such a limited number of installations available for evaluation.

DAVID C. MOFFITT, PE
Environmental Engineer/Water Quality Specialist

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