At its essence, leachate management consists of two basic steps: pump and treat. But add ash to the mix, and collection and treatment becomes infinitely more difficult. Pasco County, Fla., officials learned this lesson first-hand when ash leachate — laden with a high concentration of total dissolved solids (TDS) — began to affect the quality of effluent at the wastewater treatment plant where leachate was sent.
Determined to build a tailored leachate management system, the county has created a new model to treat ash leachate, use the reclaimed water, minimize demand on the area's potable water supplies and, ultimately, save money.
All Up in Ash
Pasco County's site monofills its ash — waste is incinerated at the county's waste-to-energy (WTE) facility before it is disposed of. While incineration saves space, the leachate produced by the ash is highly variable. On average, the leachate's chemical composition primarily is calcium chloride and sodium chloride, with a typical TDS concentration of about 37,000 milligrams per liter (mg/l). However, this does not paint a complete picture, according to the county.
TDS from Pasco's ash leachate, in fact, ranges from about 6,000 mg/l to more than 78,000 mg/l [see “Pasco County Ashpile Leachate Quality” on page 105]. And treating leachate with such high TDS concentrations can be difficult.
Oftentimes, landfills discharge their leachate into a publicly owned treatment works (POTW) facility. However, Pasco county's wastewater treatment plant (WWTP) was not up to the task — it's biological treatment process could not remove TDS. And the WWTP flow was too small to dilute the concentrated leachate, which makes up about 10 percent to 15 percent of the plant's total influent. Groundwater detection wells around the percolation ponds began to indicate elevated chloride concentrations. And the WWTP effluent became unfit for landscape irrigation and cooling water uses. The system was overloaded with TDS, and something had to be done.
Exhausting its Options
County officials, with the help of the Tampa, Fla., Camp Dresser and McKee (CDM) staff, which it hired to assist in the project, compiled a list of alternatives and weighed the options. The concentration of Pasco's ash leachate ruled out more conventional chemical and biological treatment processes, leaving the county with:
Direct disposal into the Gulf of Mexico;
Spray drying in WTE spray dry absorbers;
Evaporation/crystallization (mechanical and natural); and
Micro filtration/reverse osmosis (MF/RO).
Then, one-by-one, the list of five began to narrow.
First, the county decided that direct disposal of leachate into the Gulf of Mexico would be too controversial. The quality of Pasco's leachate is similar to the quality of concentrated seawater, but decision-makers were not willing to risk a public uproar and eliminated this option.
Then, the geology of Pasco county and its lack of a confining strata to assure the protection of groundwater resources, eliminated deep-well injection. Some suggested leachate be transported to an existing deep-well located outside the county, but officials rejected this because of costs and long-term availability. Decision-makers wanted a long-term solution, and transporting the leachate would be only a Band-Aid, they said.
|1TDS as sum of ions.|
|General note: Water analyses were balanced ionically by adjusting sodium. All values are mg/l.|
Spray-drying the leachate in WTE spray-dry absorbers was discussed at length. This would require concentrating the leachate and introducing the concentrate into the existing spray dryers used for acid gas control. But officials raised concerns about accelerated corrosion in the baghouse, adverse impacts on WTE residue handling systems and the difficulty of contracting the procedure, and finally eliminated this alternative.
Natural evaporation was not possible in Pasco county because evaporation potential is equal to precipitation. Central Florida's heavy rains would overwhelm uncovered holding ponds, thereby requiring additional leachate disposal.
Thus, the only remaining option was MF/RO and mechanical evaporation/crystallization.
A Solution in Sight
At this point, CDM evaluated permit feasibility and found that Florida's state district regulatory agency would not allow liquid brine disposal in the landfill. The engineers agreed that liquid brine, a concentrated mixture of leachate, could have negative long-term effects on landfill leachate quality and could compromise the selected treatment process. Thus, decision-makers agreed to a system that would isolate the recovered solids and remove them completely from the landfill's hydrologic cycle.
In addition to researching local regulations, CDM evaluated the cost of new storage facilities. The company designed a model to determine how much storage each leachate management scenario would require, calculating leachate production based on historical, site-specific records and weather recording station data. Further analysis showed there would be little or no cost difference between MF/RO and mechanical evaporation/crystallization systems, once the liquid brine was dried. Consequently, the county sought proposals from manufacturers of both systems.
CDM developed a request for qualifications, which Pasco County issued in the summer of 1994. After narrowing the bidders to a list of three, the county eventually chose Resources Conservation Company (RCC), Bellevue, Wash., based on the company's experience in treating inorganic wastewater streams.
CDM helped the county negotiate a contract for supply, erection and testing of the seeded slurry brine concentration process. Then, the engineers determined the optimal facility size, secured environmental permits, finished drawing plans for the plant site and monitored construction and preliminary testing.
CDM specified construction materials that were able to withstand leachate's corrosive properties. RCC responded with materials, which included 316-L stainless steel for vessels, pumps, and motors; 6-percent molybdenum stainless steel for the brine concentrator recirculation ducts; and titanium for the heat exchanger.
Unlike most leachate treatment systems, Pasco County's process requires a brine concentrator (BC) and a spray dryer system. The BC reduces leachate in volume and produces a high-quality distillate, as well as a highly concentrated waste stream (brine slurry). Leachate first is pumped from the landfill wet wells to a 2 million gallon concrete ground tank. From there, leachate is transferred to the BC feed tank, where sulfuric acid is added for pH adjustment. Sodium sulfate also is added to facilitate the seed slurry operation. And a scale inhibitor is added to prevent scaling in the heat exchanger and de-aerator.
Next, the feed is pumped through the heat exchanger and heated to near boiling. The hot feed then passes through the de-aerator, which strips carbon dioxide (CO2) and other non-condensables before the feed enters the evaporator sump.
After that, hot feed enters the flood box at the top of the evaporator through a patented brine strainer, which is designed to remove chip and prevent distributor plugging. From there, the feed is distributed evenly to the inside of the evaporator's vertical tubes to establish a thin uniform film.
Steam collects above the sump level, passes through a mist eliminator and enters a vapor compressor. Then, to raise its condensation temperature slightly above the boiling point of the brine inside of the tubes, the steam is compressed. Discharge steam condenses on the outside of the tubes and is collected in a distillate tank. The condensate then is pumped to the WTE facility to be used for boiler feedwater makeup and in the cooling tower [See the process flow diagram on page 104].
The brine slurry from the evaporator flows to an agitated spray dryer feed tank that keeps the solids suspended. Then, the mixed slurry is pumped to the atomizer in the spray dryer, where a natural gas-fueled air heater supplies heated air to the drying chamber. As it falls to the bottom of the chamber, brine is dried to a dry solids powder. Air from the exhaust fan then transfers the powder to the fabric filter baghouse. Solids exit the baghouse through a rotary valve, are discharged into a trailer-mounted disposal bag and are placed in the landfill [See Solids Discharge Diagram on page 106].
Key to the process is beginning by adding calcium sulfate (gypsum) when the BC unit is started. These gypsum solids, circulating within the brine slurry, serve as nucleation sites for calcium sulfate precipitation. Gypsum also encourages crystal growth, which occurs as the feedwater is concentrated. As the mechanical agitation of the recirculation pump breaks up precipitants and crystals, new seed material is created. Thus, the seeding process is self-perpetuating. No additional gypsum must be added [See Seeding Process Diagram on page 108].
Energy and Fuel Use
With its system in place, Pasco County now can effectively treat its leachate. An added benefit is that all electrical energy to power the treatment system comes from the WTE facility, which operates above the minimum capacity required by its Power Purchase Agreement with Florida Power Corp. (FPC), St. Petersburg, Fla.
|Analyte||TCLP Results (mg/l)||Regulatory Threshold|
The leachate treatment facility requires about 0.43 megawatts of electrical energy to operate its vapor compressor and various pumps and fans. To start the boiler and to heat the spray dryer's air, the facility uses natural gas.
Under the terms of the Power Purchase Agreement, FPC only pays the avoided cost of energy for electricity sold in excess of the county's committed capacity. The county's leachate treatment facility is powered by electrical energy that is valued at approximately $0.018/kilowatt-hour (kWh), saving the county approximately $217,000 annually when compared to retail electric energy prices.
Because the facility uses approximately 3 million Btus of natural gas per hour, landfill gas also could be substituted for natural gas. However, insufficient quantities of unprocessed Class I solid waste have been permanently disposed of at the landfill, so no landfill gas has been generated to date.
Overall, constructing Pasco's entire leachate treatment operation, which includes the 2 million gallon concrete storage tank, the 35,000 gallon per day leachate treatment facility, utility connections, solids hauling trailers and all civil site work, costs the county $4.3 million. CDM estimates annual operating costs, including utilities, chemicals, labor, allowances for mechanical renewal and replacement, and solids handling bags, will be $520,000. This means the county will pay about $0.041 per gallon of leachate disposed.
Data on the distillate and solids quality of the county's ash leachate indicate the distillate has a conductivity of 66 micro-ohms per centimeter, a pH of 7, a total alkalinity of 26 mg/l and an ammonia nitrogen concentration of 7.5 mg/l. All other analytes were nondetectable.
Previous leachate analyses have shown that metals concentrations in Pasco's ash leachate are so low that they would meet drinking water standards. But to build public confidence, CDM has conducted a toxic characteristic leaching procedure (TCLP) analysis on the dry solids. Results indicate that the metals concentrations are far below any threshold of regulatory concern [See “Pasco County Leachate Solids TCLP Analysis” above].
Daniel E. Strobridge is a vice president and program manager for Camp Dresser & McKee in Tampa, Fla. This article is based on “A Case Study in Innovative Leachate Treatment,” presented at the Solid Waste Association of North America (SWANA) WASTECON 1996. It was updated and is being printed with the permission of SWANA, Silver Spring, Md.