TRENDS 545

Robert Hauser

What's worse than heavily contaminated leachate? Ask landfill managers in Pasco County, Fla., who were facing a high concentration of contaminants plus a very high concentration of total dissolved solids (TDS). The contamination - mostly calcium chloride and sodium chloride, was found in fluids being generated from dedicated cells containing ash from a local 1,050-ton-per-day (tpd) waste-to-energy (WTE) facility.

In many cases, diluting leachate will solve pollution problems. Often, the treatment would occur in a large wastewater treatment plant (WWTP) where the effluent is a small percentage of the average daily flow. When Pasco began diverting its solid waste facility's flow to their small WWTP, it experienced elevated chloride concentrations in groundwater detection wells near the percolation ponds where treated effluent is disposed.

The county was forced to seek alternatives. A limited number of treatment and disposal process options remained:

* direct disposal in the Gulf of Mexico;

* deep well injection;

* microfiltration/reverse osmosis;

* evaporation/crystallization via mechanical and natural means; and

* spray drying in WTE spray dry absorbers.

After a preliminary evaluation, the project team led by Camp Dresser & McKee (CDM), Cambridge, Mass., and Pasco County determined that a combination of microfiltration/reverse osmosis and mechanical evaporation/crystallization were realistic options.

For municipal sanitary landfills that do not produce leachate high in TDS, the microfiltration/reverse osmosis process is effective. In essence, it separates the leachate into a high-purity, drinking-water-quality stream and concentrated leachate. The clean water is discharged to an on-site stormwater system and the concentrate's recycled portion is returned to the landfill. In Pasco County, though, cycling the concentrate back to the ashfill would increase the strength of the leachate over time, thus decreasing the process' long-term efficiency.

Pasco then opted for another solution: a brine concentrator (BC) and spray dryer system. This process reduces the volume of leachate in the BC, in turn producing a high-quality distillate and a concentrated waste stream. This concentrated waste stream then is directed to a spray dryer for production of a dry free-flowing solid material, which is sealed in poly supersacs and placed in the landfill.

This approach allows for the removal of the salts from the hydrologic cycle in the landfill. It also produces distilled water, which is used as boiler feedwater makeup. Removal of the leachate flow from the WWTP allows the reclaimed water to be used for landscape irrigation purposes, thereby reducing demand on potable water supplies.

Leachate management and treatment will continue to be a constant challenge for solid waste experts. In some cases, conventional chemical and biological treatment processes prove to be the most effective methods. In other cases, alternatives must be found.

Although the brine concentrator and spray dryer system solved Pasco's problems, it is necessary to consider all characteristics of a landfill site before determining and implementing the most appropriate and effective leachate treatment system.