Siting and Operating a Sanitary Landfill for a 120,000-Population LGU
A municipality of 120,000 people generates waste at 0.55 kg per capita per day. Waste characterisation shows 45% biodegradable (kitchen and yard waste), 25% recyclable, 20% residual and 10% special waste including used oil containers and waste batteries. The old open dumpsite beside the river is under a closure order. The LGU has identified a 5-hectare site of which 3 hectares can be used for waste placement, with a compacted in-place density of 0.80 t/m3 and an average fill depth of 6 m. The draft design includes an MRF and a windrow composting yard sized to capture 70% of recyclables and 60% of biodegradable waste, no liner, and a leachate drain discharging to a creek 150 m away. Rainfall is 2,000 mm per year. The council wants the site to serve for 10 years before buying new land.
1. Calculate the daily residual waste and the landfill life. Does the design meet the council's 10-year horizon, and what single change buys the most time?
Read the worked answer
Waste generated. 120,000 x 0.55 kg = 66,000 kg/day = 66 t/day.
Diversion.
- Recyclables: 25% x 66 = 16.5 t/day; at 70% capture = 11.55 t/day diverted.
- Biodegradable: 45% x 66 = 29.7 t/day; at 60% capture = 17.82 t/day diverted.
- Special waste: 10% x 66 = 6.6 t/day. Batteries and oil containers are not landfill-legal waste and must be segregated and routed to a hazardous waste handler, but the draft design sends them to the cell.
Residual as designed. 66 - 11.55 - 17.82 = 36.63 t/day (about 36.6 t/day).
Available capacity. 3 ha x 10,000 m2/ha x 6 m = 180,000 m3. Allow 20% for daily and intermediate cover and cell inefficiency: 144,000 m3. At 0.80 t/m3 that is 115,200 t.
Life. 115,200 / (36.63 x 365) = 115,200 / 13,370 = 8.6 years. The design fails the 10-year horizon.
Highest-value change. Diverting the 6.6 t/day of special waste drops the residual to about 30 t/day, giving 115,200 / 10,950 = 10.5 years. That single change is both a legal requirement and the item that closes the gap. Increasing fill depth from 6 m to 8 m, if geotechnically supported, would add about a third more capacity and is the next lever; higher composting capture is a third.
2. Evaluate the leachate and liner design. Estimate leachate generation and organic load, and state the minimum engineering requirements before the site can be approved.
Read the worked answer
Leachate quantity. Active area 3 ha = 30,000 m2. Rainfall 2,000 mm/year = 2.0 m/year, so 60,000 m3/year falls on the site. Assuming roughly 25% percolates through the waste, leachate is about 15,000 m3/year, or about 41 m3/day.
Leachate strength. Young landfill leachate commonly runs 3,000 mg/L BOD or more. 41 m3/day x 3 kg/m3 = about 123 kg BOD/day, comparable to the organic load of a small sewage treatment plant, discharged untreated to a creek. That is indefensible.
Fundamental objections to the draft.
- No liner. A creek 150 m away and the shallow groundwater are directly exposed. A composite liner (compacted clay plus geomembrane) and a leachate collection layer are the baseline.
- No leachate containment or treatment. The drain to the creek is an unpermitted discharge.
- Stormwater is not diverted, so clean runoff is being converted into leachate.
Minimum requirements for approval:
- Composite liner and leachate collection and extraction system.
- Leachate treatment (aerobic or anaerobic plus polishing, or controlled recirculation combined with treatment and evaporation) sized for at least the calculated volume with storm peaking.
- Upgradient and downgradient groundwater monitoring wells with a baseline sampling round.
- Perimeter berms and cut-off drains to keep clean runoff out of the waste.
- Segregation of the 10% special waste, daily cover, gas venting, and a closure and post-closure care plan with a funded reserve.
Recirculation reduces volume but not pollutant mass; it must not be used to raise the leachate mound on the liner.
3. The council argues the site is only 5 hectares and acceptable because the alternative is continued dumping. As the reviewing engineer, how do you respond, and what conditions would you attach to any approval?
Read the worked answer
Framing the response. The choice is not between this site and open dumping. Open dumping is prohibited, and the closure order on the riverside dumpsite does not make an unlined landfill in a creek's drainage an acceptable substitute. Approving it converts a short-term dumping problem into a long-term groundwater and surface-water contamination problem with an uncapped liability for the LGU.
Conditions I would attach to any approval:
- Siting and environmental clearance. Confirm the site is not on a floodplain, is not underlain by a high-yield aquifer without a hydraulic barrier, and has the required environmental compliance certificate with the stated conditions.
- Capacity honesty. Present the life calculation openly at 30 t/day with special waste segregated: 10.5 years, not 10 years on the original assumptions. Make the diversion targets (70% recyclables, 60% biodegradable, 100% special waste) binding permit conditions with quarterly reporting, not aspirations.
- Phased cells. Require each cell to be constructed, lined and closed sequentially so capital is not spent on 5 hectares at once, and so failure of one cell does not contaminate the whole site.
- Institutional backing. A 10-year solid waste management plan, a barangay-level MRF and composting programme, a closure and rehabilitation plan for the old dumpsite, and a post-closure fund.
- Enforcement. A performance bond and a clear closure trigger if leachate or groundwater monitoring shows impact.
If those conditions cannot be met at this site, the correct recommendation is to keep searching for a site and continue interim transfer and diversion, not to license an unlined landfill because land is scarce.