Poster presentation at the 2001 ASAE Annual International Meeting, 
Sacramento, CA July 29-August 1, 2001 

Paper Number  012298
Session 128 
Poster #142

A Low-Cost Digester to Control Odors at a 120,000 Head Hog Farm

Mark Moser
Resource Conservation Management, Inc.
PO Box 4715
Berkeley, CA  94704
Phone 510-658-4466
FAX   510-658 -2729
rcmdigesters@att.net

Poster presentation at the 2001 ASAE Annual International Meeting, Sacramento, CA July 29-August 1, 2001 

ABSTRACT

A single cell heated covered lagoon was designed to treat manure for odor control from 120,000 pigs in South America.  The system cost $1,200,000.  The system started up readily and is producing an average of 530,000 ft3 of biogas/day.  The system benefits and methane reductions are described.   A summary table is presented to allow comparisons of costs. Biogas recovery and use in boilers or engine-generators is discussed for the farm.  Start up and operational lessons learned are presented. 

Keywords:   Biogas, methane, odor, anaerobic digestion, digester, covered lagoon, nutrient management, pathogens

Introduction

Anaerobic digestion is more extensively used outside of the US where treatment of animal waste has been a concern for a longer time.  An anaerobic digester is a vessel designed to retain decomposing manure for sufficient time at the designed operating temperature to allow the growth of methanogenic bacteria in a “steady-state”.  Electricity and heat production are direct benefits of anaerobic digestion. The effluent of a digester has an earthy smell with some ammonia present. The first dairy digester systems in the US were installed principally to produce energy during the energy crisis.

The first pig manure digester systems in the US were installed principally to control manure odors. Today, farm motivation worldwide for building and operating anaerobic digesters has expanded from direct energy benefits to include key non-energy benefits such as: odor control, improved manure handling, mineralization of organic nitrogen, weed seed destruction, pathogen reduction, reduction of greenhouse gas release, and byproduct production such as digested dairy solids. 

Complete Mix Digesters

Complete mix refers to a design approach toward a goal rather than literal complete mixing.  Complete mix digesters are used to treat waste with 3 to 10% total solids with adequate volatile solids to produce enough biogas (60% methane, 40% carbon dioxide) to maintain digester temperature.    These units are heated to maintain a high rate of bacterial growth. Intermittent mixing or stirring with pumped gas or water or mechanical means is used to improve the contact between substrate and microorganisms thereby promoting decomposition of the substrate.

A heated digester should represent a cost savings over using an ambient temperature lagoon designed to Natural Resources Conservation Service (NRCS) or American Society of Agricultural Engineers (ASAE) standards to perform the same level of treatment for odor control.  Ambient temperature lagoons would be 10-20 times larger than these digesters to perform the same function.  The design and operation goal of the subject digester is consistent biological stabilization of waste and odor control rather than optimization of biogas production and use while stabilizing the waste.

Facilmente Engordas - 120,000 Head Pig Finishing Facility, South America

Background

Engordas has 120,000 hog capacity in 120 buildings.  The site is operated as an all-in-all-out grow-finish facility.  Approximately 90 buildings were built with flushed manure systems and 30 were constructed with pull plug manure collection.  All wastes were originally drained to a storage facility which was the dammed upper portion of a small watershed drainage.  The storage facility has almost 88 acres of surface area but is predominantly shallow, less than 10 feet average depth.  Quite a bit of shallow sloped land was included in the storage.  The facility was sized for about 6 months of volumetric storage. Odor from the manure storage pond was not acceptable to the owners, workers, neighbors, or government officials.  In addition, seasonal pumping of manure exposed large areas of perimeter beaches that served as breeding grounds for copious quantities of flies.

The owners committed to resolution of the problem, conducting a world-wide solicitation for designers with demonstrated cost effective technology. The farm owner chose Resource Conservation Management, Inc. to design an innovative, low cost, heated, mixed covered earthen lagoon digester based on similar success of the approach that was demonstrated at Apex Pork in Rio, IL on an 8,900 head finisher facility.

Manure Collection and Transfer

Digester

A bank-buried insulated floating cover is installed on the 105 x 105 x 9 meter (346 x 346 x 29.7 ft) lagoon which can hold roughly 20 days worth of manure (32,000 m3 approximately 8,000,000 gallons).

Digester Heating

The digester is operated at 35 degrees C and can vary up to 4 degrees C.  Digester is heated with an external exchanger.

Digester Mixing

The digester is mechanically mixed.  Pump mixing is continuous.

Gas Production

The digester produces enough methane to fuel a boiler to heat the digester and flares unused methane.    Gas production has varied between10,200 m3 to 15,500 m3 (360,000 to 550,000 ft3/d) since the digester startup.  After 2 hydraulic retention times with the farm fully populated, the digester output averaged 530,000 ft3/d over a 2 month period.  Gas production fluctuations are expected due to fluctuations in farm pig population.

Digester Effluent

Stabilized digester effluent flows to the storage pond. 

Digester Operations

The digester was started up over a 4 month period in ending in November 2000. Flare operation began in late December with flaring of 150,000 ft3 of biogas. By late January 2001 the system was producing 5,600 m3  (200,000 ft3) of biogas per day. The unit was running well when the pig population was brought up to farm capacity. 

The digester maintained temperature during the coldest times of the year. The digester has been through 3 grow-out cycles without problems.  The system is simple to operate and a single operator is charged with operating the system.  The main work is checking the system operations readings. 

Construction Costs

The costs for construction are shown in the appended table.  Most equipment was imported and these costs are included in the cost estimate.  In-country construction costs are remarkably similar to US construction costs as virtually all construction materials and equipment are imported, thereby offsetting labor cost savings.   The capital cost is approximately  $10.06/pig of capacity.  The system life should be 15 years. Assuming no interest charges and 2.5 turns per year the cost per pig produced would be $0.27.

Operation Costs

Operations are routine at this time.  Approximately 1.5 men are working at  the digester, though their main work is pumping manure from buildings to the digester and from the digester to storage.  Assuming this facility has 2.5 turns per year, the cost per pig shipped is approximately $0.25.

Results

After 8 months of operation the digester is healthy and operating well.  It is maintaining its heat level and the storage no longer emits malodorous gases. The primary benefits to the owners are odor reduction of stored and field applied manure. There have been no odor complaints since 4 months after the digester was started up. Compliance observations by regulators continue, but the government participated in an open house 6 months after startup to demonstrate their support of the odor control solution. The digester is able to accommodate the variation in manure loading rate that results from all-in-all-out operation.  The attached figures present operational information including  influent flow, biogas flow and CO2 variations.

Lessons Learned

Several lessons of this project were: 1) Low cost, in-ground, heated, mixed, floating cover digesters can meet farm environmental needs; 2) Large single tank, anaerobic reactors are possible on farms; 3) Finishing farms with flush manure collection have the options of heated anaerobic digesters.

Future Options

The owner would like to generate electricity for his operations.  There is the possibility to install about 1.5 megawatts of electric capacity.  Equipment costs and benefits have been developed.  Equipment specifications have been developed.  Negotiations with the power company have begun.

The success of the project in meeting its environmental goals led the owner to begin another project to retrofit other farms with RCM design anaerobic digesters.

RCM Digesters
PO Box 4716 - Berkeley, CA 94704 - 
(510) 834.4568 - FAX: (510) 834.4529 
contact@rcmdigesters.com 
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