LANDFILL: Bioreactor Landfills: But Does It Save Money?

Prentiss Shaw and Amy Knight

Amidst all the buzz about wet tomb landfills, landfill operators want to know whether the long-term benefits of recirculating leachate through their landfills will outweigh the initial costs of installing recirculation equipment.

Put simply, how will implementing a bioreactor system affect the bottom line?

Research suggests that, while initial installation costs can be high, operating a bioreactor landfill saves money in the long-term by recirculating leachate instead of treating it or disposing of it. During the life of a 50 acre, six cell landfill, a bioreactor that increases waste density by a modest 200 pounds per cubic yard (lbs/cy) can save $7 million. At densities of 1,800 lbs/cy, compared to the average dry tomb landfill waste density of 1,200 lbs/cy, savings may be as high as $12.6 million. Figures like these are making proponents of dry tomb landfills reconsider their opinions.

The costs of installing leachate recirculation devices and enhanced landfill gas collection systems are the bulk of bioreactor expenses. For an 8-acre cell with a total waste height of 100 feet, bioreactor construction costs are estimated to vary from $160,000 for horizontal trenches, to $640,000 to construct vertical injection wells.

In addition to construction costs, landfill owners should anticipate a variety of operating costs resulting from increased gas generation, equipment power and maintenance, and permitting fees. Since these costs vary regionally, they are difficult to estimate.

Treating leachate for a 10-acre, closed landfill cell can cost approximately $700 per year to $8,000 per year. For an active, 10-acre landfill cell, treatment/disposal costs range from $54,750 per year to $602,250 per year. Through recirculation, these costs can be reduced significantly.

Additionally, recirculating leachate increases waste densities, effectively increasing airspace, which extends the landfill's life and adds to long-term tipping revenues. On average, recirculation produces landfill waste densities of approximately 1,200 lbs/cy, with reported densities as high as 1,800 lbs/cy. Assuming a tipping rate of $40 per ton, a landfill that accepts 500 tons of waste per day (tpd) could increase daily revenues by $6,667, and a landfill that accepts 5,000 tpd could increase daily revenues by $66,667, using biostabilization.

More airspace also can increase revenues in subtle ways. While biostabilization extends the older cells' life, money that would have been spent to construct new cells can be deferred and earn interest. At an interest rate of 8 percent, the net interest payment on a fully amortized $4 million loan would be approximately $300,000 for a one-year loan and $824,838 for a three-year loan.

Regulators now require maintenance and monitoring of closed landfills until the waste mass is stable. This post-closure period can last as long as 30 years for a conventional landfill, but research suggests that a bioreactor landfill can reduce this period to 10 years. Thus, while post-closure costs for a 50-acre conventional landfill are approximately $18.7 million, the costs to monitor a 50-acre bioreactor landfill are estimated at approximately $10.9 million. As more regulators begin to accept the viability of bioreactor landfills, these cost scenarios become possible.

To defray post-closure costs, many facilities establish escrow accounts during the landfill's operational life and divert a portion of the tipping fee to this account. Consequently, the longer a landfill can remain open due to increased airspace, the more money an escrow account will accrue. A robust escrow account ensures financial stability long after the landfill closes.

For example, a conventional 10 million-cubic-yard landfill with a waste density of 1,200 lbs/cy and an escrow funding rate of $2.50 per ton would require $15 million in escrow to pay for post-closure monitoring. Because it uses space more efficiently, a bioreactor landfill with the same specifications would save from $2.5 million to $7.5 million by reducing the funding rate, depending on waste densities.

As bioreactor technologies continue to advance, these economic benefits likely will increase, spelling good news for operators' bottom lines.