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Langerwerf Dairy Digester Facelift

What We Found When We Took Apart a 16 Year-old Dairy Plug Flow Digester

Mark A. Moser^a and Leo Langerwerf^b
aResource Conservation Management, Inc.

P.O. Box 4715
Berkeley, CA 94704
^bLangerwerf Dairy, Inc.
1251 Durham-Dayton Highway
Durham, CA 95938

Abstract

Annual digester maintenance has been estimated at up to 8% of capital cost. This project found that the long term annual digester maintenance cost was less than 1% of capital cost. The findings will reduce uncertainties that have limited adoption of digesters. Langerwerf Dairy is a 400 cow dairy in Durham, California with a plug flow anaerobic digester that has been in operation continuously since 1982. The dairy received matching grant assistance from the Western Regional Biomass Energy Program (WRBEP) to refurbish the digester and document the process. The AgSTAR program provided technical assistance. The protective greenhouse was disassembled, gas bag removed, floating crust removed, manure pumped out, heating system examined, and settled solids removed. The digester was basically in good condition. Major costs were removing floating and settled solids, and new greenhouse parts and gas bag. The digester was refurbished and returned to service. Findings include: 1) hypalon gas bag material degraded; 2) floating crust accumulated to an average 4 feet thickness; 3) sand accumulated to an average depth of 5.5 feet; 4) a dairy plug flow digester appears to accumulate only 1% of the volume of solids that would be expected in an anaerobic lagoon; and 5) there was minor corrosion of concrete and steel. Further detail including, procedures, costs, and findings, are included.

Keywords: Biogas, methane, odor, anaerobic digestion, digester, digester maintenance, digester sludge, plug flow, nutrient management

Project History

Langerwerf Dairy is a 400-cow family run dairy in Durham, California with a plug flow anaerobic digester that has been in operation since 1981. The farm received a matching funds grant from the Western Regional Biomass Energy Program (WRBEP) to assist in purchasing materials necessary to refurbish the digester, to document the process and to publish a report of findings on the lifecycle condition of digester components. At the time of refurbishing, the digester was 16 years old and needed some component repair and replacement. A Caterpillar G3306 engine generator has operated about 90% of the project lifetime, averaging about 40 kW output. In 16 years of operation the digester has produced about 120,000,000 ft3 of biogas (72,000,000 ft3 of methane) and the engine converted the biogas into 5,000,000 kWh. Approximately 23,000 yd3 of digested fiber have been sold. The original digester, generation and solids separation system cost $200,000. The value of the electricity produced is about $350,000; the value of the digested fiber sold has been about $138,000; about $75,000 in farm hot water was recovered from the engine; and $135,000 was saved in lagoon cleanout costs. The farm has spent about $160,000 on operation and maintenance including this project. The estimated return for the 16 years of operation is approximately $540,000.

Digester Construction

A 8-foot high perimeter chain link fence was taken down. The protective greenhouse (2-layer clear plastic over galvanized steel hoops) enclosing the digester was disassembled and hauled away. The gas collection cover and was removed and disposed of. These tasks were performed over a one-week period.

A 3 - 5 foot thick, hard floating scum was found. The scum would easily support the weight of a man. The scum was removed by an articulating trackhoe with a skilled operator. The operator scooped the scum up and deposited it directed into farm dump bed trucks. About 350 cubic yards of matted hard scum were hauled to the fields and applied. The scum removal required about 2 days and about 4 people plus the trackhoe operator.

The remaining digesting slurry was pumped out, with about 40,000 gallons reserved in a nearby tank for digester startup. The black iron hot water heating system was examined and found to be in good condition. Five feet of settled solids, mostly fine sand with some organic material was found covering the bottom and part of the heating system.

Approximately 400 yards of settled sands were in the digester and approximately 330 yards were removed using a hydraulic mining technique. High-pressure hoses using recycled digester water were used to wash settled solids to pump that pumped the mixture to a settling basin near the digester. Clarified liquid flowed back to the digester.

Cleaning out the solids required about 10 days because several different approaches were tried, before arriving at the workable solution. The work would have required 4 men for 5 days even if the hydraulic mining with recycle had been initially used. Once the digester was emptied all components were inspected for serviceability.

Components and Materials after 16 Years of Operation

Gas collection cover
The hypalon gas collection cover material degraded and failed due to UV weathering. UV weathering caused pinholes and subsequently biogas and water infiltrated into cover reinforcing creating larger holes and leaks.

Gas collection cover attachment
Black flat bar steel, angle iron and galvanized bolts were used to attach the cover to the concrete tank. Sheet metal capping was installed on the top of the digester wall. The exposed wall top galvanized sheet metal cap rusted away. These materials were on the atmosphere side of the gas collection cover and exposed to the same moisture and hydrogen sulfide conditions as the greenhouse hoops. Additionally some manure had run along the inside edge between the cover and the digester wall at some time and remained in contact with the bolts. At least a dozen bolt heads had corroded enough to require drilling out. Some of the flat bar steel had corroded because it was left in contact with manure and air. Most of the angle iron was rusty but not corroded significantly. New hardware was used for cover reattachment.

Concrete
Some concrete corrosion was found in the same areas as the cover attachment corrosion and is attributed to manure that had run along the inside edge of the concrete and been exposed to air as it dried. The manure decomposed forming acids that etched away one eighth to one quarter of an inch of concrete over a 40 foot length of concrete sidewall. The corrosion presented no problem with the digester operation.

Liner material
Hypalon material was also used for the digester liner. The material was judged to be suitable for continued service. It was aged and grainy in spots but did not exhibit the holes or liquid infiltration. No buildup of struvite was found.

Heat exchanger and pipe
The digester heat exchanger was constructed of black steel. It was found partially buried in sand and judged to be not fully effective. Upon removal of the settled sand no external corrosion was found.

Gas collection pipe
No degradation of any PVC plastic including the gas collection pipe was found. The gas intake T where the cover rested was slightly deformed, probably due to 16 years of the cover resting on it and being exposed to high temperatures. There was some accumulation of manure solids evident in the gas line probably from startup foaming.

Greenhouse component
Galvanized greenhouse support hoops were corroded at unprotected welds. The corrosion can be attributed to normal condensation mixed with some hydrogen sulfide. The greenhouse plastic was in need of replacement after 4 or 5 years.

Refurbishing and Restarting the Digester
A pipe coupling in the heating system was broken during the cleanout process and later was repaired. The digester was refilled on 10/22 with new and old manure. Heating began on 10/23 with the engine-generator running on propane. The new digester cover was installed using new steel and bolts on 10/28. On 10/29 the digester temperature was at 85 degrees and 5% new manure was added. The digester gas meter was installed and the biogas gas tested in flame test the same day. The flame was consistent and had good color characteristics. Late on 10/29 the biogas was tested in the engine. Full power was demonstrated and therefore the engine was set at 20 kW and run continuously. Table 1 shows the log of the startup of the digester.

Table 1 . Langerwerf Dairy Digester Startup Log

By 11/1 the engine was running full time at 30 kW based on a production of almost 20,000 ft3/d. The greenhouse was installed mid November to protect the digester during winter weather. On 11/30 engine output reached 40 - 45 kW at continuous operation.

Current Operations

Table 2 shows the farm electricity use before and after the project directly from PG&E invoices. In September 1998 the digester was shut down, showing the real farm requirements for electricity. In October 1998, the digester was heated with propane fueling the engine at low output and reduced the electricity use. November 1998 is the startup month where gas production increased and was burned for electricity production, resulting in a decline of

electricity use to 109 kW/d. The digester electricity production saved the farm $850 in October alone. The 1998 purchases can be compared with the 1997 purchases and it can be seen that even with just startup operation, the farm is buying less electricity that in 1997. At the same time, the digester system produced about 1000 surplus kWh in November 1998 that were sold to PG&E for $356.27. The farm was not selling nearly as much electricity in 1997.

Table 2. Electricity Purchases Comparison 1997-8

Project Expenditures

The project was completed on time and on budget. Table 3 summarizes the budgeted versus actual expenditures. The situation found upon opening the digester necessitated altering some strategies and planned work. Hired labor was substituted for a contractor when it was obvious that mantime was more necessary than skilled construction assistance. Savings were used to spend more money on rental equipment for removing scum and settled solids. A trackhoe to remove scum saved money which was then used to hydraulically mine solids. AgSTAR assistance substituted for some of the planned farm personnel time. AgSTAR personnel suggested the recycle settling ponds, set up the cover for installation and worked with the cover installation crew.

Table 3. Costs of Refurbishing the Digester

Conclusions

The materials originally selected for use were very satisfactory. We are very pleased with the condition of materials in the digester. The cover material did not last 20 years as projected by the manufacturer. Gas collection cover materials may only be serviceable for 10 years, however the cost of a new cover every 10 years does not materially affect the profitability of the digester. The only unusual finding is that the digester had been operating successfully. The amount of floating scum and settled solids reduced the usable volume of the digester by about 66%. Only 1/3 of the digester was actually hosting active digestion. However, the lack of retention time had caused a drop off in performance, leading to the recognition of the need for digester cleanout and refurbishing.

Observations for Industry Consideration

The accumulation of solids and scum in a digester are expected. However, a dairy plug flow digester appears to accumulate only 1% of the volume of solids that would be expected in an anaerobic lagoon. The following section compares the volume of solids found in the plug flow digester with the estimated sludge accumulation volume in an anaerobic lagoon. It may not be fair to compare a digester and a lagoon because a lagoon may also provide storage. However, if a separate storage facility for digester effluent requires no sludge volume, because all but 6 inches of sludge is assumed to be pumped out each time the storage facility is emptied.

Approximately 10,000 ft3 of scum and 8,000 ft3 of settled solids were found to have accumulated in the Langerwerf plug flow digester over 16 years. Occasional mixing to reduce scum had been attempted with minimal success from years 14 - 16. For purposes of estimation - we can say that about 20,000 ft3 of material accumulated over the life of the plug flow digester.

The NRCS Field Waste Management Handbook (National Engineering Handbook, Agricultural Waste Management Field Handbook, USDA, Soil Conservation Service, April 1992) page 10A-3, contains the method used to calculate solids accumulation in anaerobic lagoon treatment systems. An anaerobic lagoon receiving the waste from 360 cows would accumulate 2,300,000 ft3 of sludge over the same 16-year period as the cleanout interval of the plug flow digester. A recent revision to Practice Standard 359-1 (Natural Resources Conservation Service Conservation Practice Standard, Waste Treatment Lagoon Code 359, January, 1998, Revision 4) distributed through the North Carolina State NRCS office, reduces the dairy sludge accumulation volume and for this comparison would only require 1,500,000 ft3 of sludge storage volume.

Therefore, in a 16 year cycles, 20,000 ft3 (740 cubic yards) of material accumulates in the plug flow digester and 2,300,000 ft3 (85,000 cubic yards) of material accumulates in a lagoon and must be managed. One can easily calculate that cleaning the digester and hauling off material in 10 yard dump trucks would require 74 truck loads. The same maintenance for a lagoon would require 8,500 truckloads or some other rather large investment to remove sludge. Under North Carolina guidance only 5,555 truckloads would need to be moved.

If a farm has the option of a digester and storage or an anaerobic lagoon, the farm should be aware that sludge storage is not required for a digester while a lagoon will be much larger than the digester with 85,000 yards of excavation required just for sludge storage. Eventually the farm with the lagoon will have to manage 5,000 to 8,000 dump truck loads of sludge that a digester does not accumulate.

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