55 GAL DRUM

OPERATION

Incoming Waste

A system can be designed to digest a wide variety of organic wastes, from kitchen scraps to sewage, to livestock manure, to industrial wastes. The ideal feedstock is a 6-8% slurry with a Carbon to Nitrogen ratio of about 30:1. Incoming waste material should be macerated, and as close to the operating temperature (95 degF) of the digester as possible. The small scale system described below will handle the toilet wastes produced by a family of 6, each flushing a 1-1/2 Pint/flush toilet 5 times per day.

Gas Handling:

55 GAL DRUM BIOGAS DIGESTER

Gas can be stored in low pressure gas bags (i.e. truck tire inner tubes, etc.), rigid tank(s) with floating cover and water seal, compressed and stored in pressure tank, and/or burned as it is produced, (minimizing storage requirements). For safety reasons, it is recommended that the gas be burned as soon as possible, avoiding the requirement to store and handle larger quantities of flammable gas.

The gas produced typically consists of about 30% CO2 and about 60-65% methane, depending on the content of the wastes. Small amounts of hydrogen, hydrogen sulfide, and nitrogen gas will also be produced, as well as water vapor.

One use of this gas is to heat domestic hot water in a gas-fired tank-type water heater installed between the cold water inlet and the conventional (backup) water heater. A small, weighted inflatable gas bag will be used to collect up to an hour's worth of gas (i.e. about 5 cu ft at design conditions), and to provide required pressure for proper burner operation.

Gas burner should be as small as possible, with intermittent ignition with continuous retry and maximum lock-out time. Sufficient hot water storage capacity should be provided to make use of all available energy without having to store flammable gas. This may require addition of one or more insulated storage tanks piped together, and a small circulator pump and controls.

Digester Effluent:

Effluent from the digester will be returned to the conventional backup sewer system. A hydraulic loading rate (total liquid throughput) of about 60 gallons/ day is assumed. Estimated solids loading will be about 35 lbs/day. Assuming that volatile solids will be reduced by about 60-70% in the digester, additional volatile solids entering the sewer system will be about 35 - (35 x .65) ~ 12-15 lbs/day.

Digester Sludge:

The volume of sludge solids accumulating in the digester will depend on the digestibility of the influent material and the extent to which digester contents are mixed (i.e. kept in suspension and discharged with effluent), or allowed to settle. Tanks are designed to facilitate sludge removal (e.g. quick disconnect fittings provided for connection to vacuum pump, etc.). To reduce solids loading on backup sewer system and recover sludge solids as a valuable soil amendment, a settling tank can be installed in the line between the effluent overflow and the sewer system

Gas Bio-digester Informationand Construction ManualFor Rural Families

Fundación Cosecha Sostenible HondurasF U C O S O HOficina de la Coordinación Nacional

Table of Contents

I. What is a bio-digester?II. Considerations before building a bio-digesterIII. Materials and Instructions for installing afamily sized bio-digesterIV. Operating and maintaining a bio-digesterV. Common questions & problems

Written and compiled by Laura Brown, November 2004. This report is based on information gathered by Bruce Maanum, Stuart Green, and the agricultural extensionists employed by FUCOSOH. FUCOSOH has provided technical and financial support for the construction of 25 bio-digesters in rural Honduras.

Pressurereleasesystem

Plastic digesterbagWasherAssemblyPrincipalgas releasevalveGas controlvalveGASOutletInletManure & water

If you will need to transport the bags to a different location, fold and securethem so the elements or the heat of a motor will not damage them.

Assembling the Gas Outlet

3. Place one washer and one gasket on the male adaptor (as shownbelow) and thread through the inner side of the plastic bags. Push themale adaptor through the hole. Assemble the remaining washer andgasket on the outside of the bags. Apply PVC cement to the maleadaptor and secure the female adaptor firmly. Tighten with a wrench ifavailable.

4.

Attach the long solid PVC pipe to the washer assembly with cement.NOTE: Unless otherwise indicated, all PVC attachments in the bio-digester should be cleaned sandpaper or a knife to ensure a propercement seal.

Installing Inlet and Outlet Buckets

5. Lay your bio-digester carefully in the trench being sure that the gas outletis centered. Cut out the bottoms of the 5-gallon buckets and sand or filethe edges to remove any sharp areas that may damage the plastic. Slideone side of the tied digester bag through the bottom of one of the 5-Gallon buckets Leave about 1 ½ -2 feet of plastic coming out the top ofthe pail. Carefully fold the ends of the bag over 2-3 times and tie off eachend with straps of rubber.6. Push the bucket into the trench so it sits at about a 45-degree angle. Atthis time it may be necessary to dig ramps at the entrance or exit of thedigester to accommodate the angle of the buckets. You may alsoconsider digging a terrace that will allow for easy accessibility to theentrance bucket. Repeat on the other end of the bag.7. To secure the buckets pound stakes on each side of the inlet betweenthe pails and the bag. Stakes should be placed a distance slightly smallerthan the diameter of the bucket so the bucket will fit very snugglybetween them. Be very careful not to puncture the bag.

Positioning and Fillingthe Digester

8. Before filling your digesteryou must ensure that it sitssnuggly in the trench withoutfold or wrinkles. Any folds orwrinkles that remain whenthe bag is filled with watermay chafe and form holes.This may be accomplished inseveral ways:

Above: Installing the washer assembly in the digester bag.

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If a diesel motor isavailable the digestermay be filled withexhaust. Attach oneside of a flexible hose

Above: Bio-digester bag is hanging between two trees while filling with water.

(do not glue) to the PVC pipe extending from the washer assemblyand the other to the exhaust source. Fill just until the bag is smooth.

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If a diesel motor is not available thread a long piece of rope throughthe digester bag and secure ends to nearby trees. The bottom of thebag should sit on the floor of the trench and the top should be raisedslightly. Fill the bag with water as described below.

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Fill the bag with air using a small hand or foot pump.9. Attach one end a flexible plastic tube (do not glue) to the PVC pipe fromthe washer assembly and the other to a spigot. Fill the digester to 60%-75% capacity with water.NOTE: Water used to fill thedigester must be clean andhave neutral pH (not tooacidic or basic). Ask yourextensionist if your watersource is suitable for fillingthe digester.

Completing Inlet andOutlet Tubes

10. Untie the ends of thedigester bag and fold theremaining plastic over thetop of the bucket. Reacyour hand through tosmooth the plastic alongthe insides of the bucketSlide another plasticbucket bottom end firstinside the first bucketthrough the plastic tube.The plastic should besandwiched between thetwo buckets. Wraprubber ties around thebucket to secure theextra plastic.h.

CORRECT INCORRECT

Below: Placing and securing Inlet and outlet buckets. The bottom of the bucket should be submerged at least 6 inches below the water level.

Below

: Threading bags through the plastic bucket inlet tube. Only one bucket was used in this model.

11. If necessary reposition the angle of the inlet and outlet tubes so thebottoms are well below water level and fluid can easily flow out of thedigester. Tie a long piece of rope between the two vertical stakes to holdthe tubes firmly in place.VERY IMPORTANT: Bio-digesters rely on water seals to maintain an oxygenfree environment. If oxygen is allowed to enter the bacteria that produce gasin the digester will die. To ensure a good seal inlet and outlet tubes must besecured so the bottoms are AT LEAST 6 INCHES below water level.

Assembling the Pressure Release Valve

12. Begin assembling the gas valve by rolling the steel screen or steel woolinto a ½” tube. Push the tube into the PVC “T” as shown.

No. Over time sediment can destroy the bag, reduce the production of gas, andreduce the gas storage capacity of the bag. Be sure that the charging mixture isfree of heavy hard materials and that inlet and outlet tubes are not blocked by