Covered Anaerobic Digester for a 500-Head Piggery: Sizing, Gas Use and Effluent on 5 ha of Pechay
A 500-head finishing piggery in Bulacan uses a flush system and produces about 2.2 kg of fresh manure per head per day, of which 90% is collected. The fresh manure is 20% total solids, and 80% of the solids are volatile. The operator wants a covered anaerobic digester to stop complaints from a nearby subdivision, cut cooking fuel costs and use the effluent on 5 ha of pechay. Design assumptions on hand: biogas yield 0.35 m³ per kg of volatile solids destroyed, 60% methane, mesophilic digestion with a 25-day hydraulic retention time, and slurry diluted to 8% total solids for pumping. The farm spends about PHP 24,000 a month on LPG, roughly 400 kg, for the kitchen and a 5 kW aerator for the old lagoon. Site depth is limited to 3 m because of the water table, and groundwater temperature drops to about 24°C in the rainy season.
1. Size the digester and estimate daily biogas production. Check the organic loading rate against a mesophilic limit and comment on whether the 3 m depth restriction is a real constraint.
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Manure collected. 500 head × 2.2 kg/day × 0.90 = 990 kg fresh per day.
Solids. TS = 990 × 0.20 = 198 kg/day. VS = 198 × 0.80 = 158 kg/day.
Biogas. 158 × 0.35 = 55.4 m³/day, of which 60% is methane, about 33 m³ CH4/day.
Hydraulic load. Diluted to 8% TS: 198 / 0.08 = 2,475 kg/day, essentially 2.48 m³ of slurry per day.
Digester volume. Working volume = 2.48 × 25 = 62 m³. Add about 20% for gas headspace and freeboard, giving roughly 75 m³ total.
Organic loading rate check. OLR = 158 kg VS/day ÷ 62 m³ = 2.55 kg VS/(m³·day). That is inside the usual 2–3 kg VS/(m³·day) band for mesophilic digesters but not comfortably so - it leaves little room for a surge in flushing water or a batch of spoiled feed. If flushing practice is inconsistent, use a 30-day HRT (about 75 m³ working volume, OLR 2.1).
Depth restriction. At a 3 m limit with 2.5 m of liquid depth, 62 m³ needs about 25 m² of surface - roughly 5 m × 5 m. That is entirely feasible; the restriction rules out a deep tank, not a digester. What it does affect is ground heat loss, since a shallow tank has a high surface-to-volume ratio in contact with 24°C groundwater, which leads directly into the temperature problem below.
2. Quantify the gas in useful terms and decide how the farm should use it. Should the surplus go to a generator, and what does the LPG bill tell you?
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Energy value. Biogas at 60% methane has a lower heating value of about 21.5 MJ/m³. Daily output = 55.4 × 21.5 ≈ 1,190 MJ/day, about 330 kWh of thermal energy per day.
LPG equivalent. LPG at about 46 MJ/kg: 1,190 / 46 ≈ 26 kg/day, or roughly 780 kg a month. The farm currently uses about 400 kg a month. So the digester could supply about twice the kitchen demand, and the LPG bill of PHP 24,000 would essentially disappear - with a surplus left over. That surplus is the real decision point: flaring it is a waste, and the alternative uses are a small generator, a brooder or water heating.
Generator case. At 28% electrical efficiency, 1,190 MJ/day gives about 92 kWh/day, roughly enough to run a 7–8 kW set for 12 h, which would offset the 5 kW aerator and some lighting. But be conservative: derate for rainy-season digester temperature, digester start-up time, and parasitic loads (the slurry pump and possibly heating). A realistic first-year average is nearer 60 kWh/day.
Recommendation. Size the gas train for the kitchen first (a gas-fired water heater plus cooking burners), because that displaces the most expensive fuel per unit of energy and gives an immediate, visible return. Add the generator only after a stable three-month gas log shows a reliable surplus, and keep a flare or a pressure-relief burner because a covered digester that cannot vent safely is a safety hazard. Also confirm the burner jets are sized for biogas, whose volumetric energy density is only about half that of LPG - an LPG jet will give a lazy, unstable flame.
3. The effluent must go onto 5 ha of pechay. Using a nitrogen balance, decide whether that land can absorb it, and state the practical and regulatory risks.
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Volume. About 2.5 m³ of digested slurry per day, roughly 900 m³ a year.
Nitrogen. Fresh pig manure carries roughly 0.55% N on a fresh-weight basis, so 990 kg/day × 0.0055 ≈ 5.4 kg N/day. Assume 85% of the N stays with the liquid effluent after separation: about 4.6 kg N/day.
Land balance. Pechay needs roughly 60–120 kg N/ha per crop of about 45 days. Over 45 days the effluent delivers 4.6 × 45 ≈ 207 kg N, which spread evenly over 5 ha is about 41 kg N/ha - comfortably inside the crop requirement. Hydraulically, 2.5 m³/day over 45 days is about 112 m³, or 22 mm over 5 ha, which is trivial for the soil.
So yes, in principle - with conditions. (1) Apply it to only part of the area at a time so that no single block is over-loaded; 41 kg N/ha averaged over 5 ha means some blocks would get three or four times that if the applicator concentrates on the nearest field. (2) The N is largely ammoniacal: incorporate or irrigate in promptly or a large share volatilizes, and do not apply to leafy vegetables within the harvest interval because of pathogen risk. (3) Watch salinity and ammonia burn on seedlings, and check soil EC periodically. (4) Never discharge effluent to a creek or drainage canal - check the current DENR general effluent standards (BOD, TSS, pH) before any discharge point is contemplated, and prefer zero discharge. (5) Screen and settle solids before pumping, or the emitters and furrows will clog.
The honest caveat. A digester reduces organic load but does not remove nitrogen, phosphorus or pathogens. It converts an odor and methane problem into a nutrient-management problem, and the nutrient plan has to be written before the first tank is poured.
4. Three months into operation during the rainy season, biogas output drops about 40% with what looks like the same feed. Give your diagnostic order and the likely causes.
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Start with the one variable you already know changed: temperature. Groundwater at 24°C against a mesophilic optimum near 35°C slows methanogenesis sharply; a 10°C drop can easily cut gas production by a third or more, and that alone could explain the drop. Shallow tanks with a high surface-to-volume ratio, which is what the 3 m depth limit forces, lose heat fast. Check the digester temperature at three depths before touching anything else.
Then check the feed and the water. In the rainy season, flushing volumes usually rise: more water means a shorter effective HRT and diluted substrate. Compute the actual daily slurry volume against the design 2.48 m³ and the actual HRT against 25 days. Also check whether wash water, rainwater runoff or disinfectant has entered the pit - detergents and antibiotics are common causes of a sudden die-off.
Then the process chemistry. Measure pH and volatile fatty acids against alkalinity. Falling pH with rising VFA means acidification from overloading, underfeeding, or a toxic shock; rising pH with falling gas can mean ammonia inhibition from too much undiluted manure. A VFA-to-alkalinity ratio above about 0.3 signals trouble.
Then the physical plant. Check for a floating crust or scum layer, blocked inlet, gas leaks in the line and fittings (a soap test), water in the gas line, a stuck condensate trap, and pressure loss across the desulfurizer. A gas leak is easy to mistake for a biological problem.
Order of action. Temperature, then actual feed and dilution, then pH/VFA, then leak test. Do not respond to a 40% drop by adding more manure to "make more gas" - that is the fastest way to sour a digester that is already stressed.