R.M. Towill Corporation
09/01/00 - 03/31/01
The Sand Island Wastewater Treatment Plant (SIWWTP) is the largest treatment plant in Hawaii. At SIWWTP, an average of over 70 million gallons per day of sewage is treated to the enhanced primary level via flotation clarifiers and then discharged through an 84-inch ocean outfall located 2 miles offshore at a depth of 240 ft. The solids removed from the sewage form a sludge which is currently thickened (gravity), subjected to thermal conditioning (380 degrees F, 330 psi), and then dewatered (centifugation). The dewatered sludge is currently disposed in the Waimanalo Gulch landfill. Previously, the dewatered sludge was incinerated at SIWWTP in two multiple hearth furnaces (MHFs) and only the ash was disposed at the landfill. The incinerators have not been operated for several years due to high operating costs. The City and County of Honolulu has been investigating various methods for processing of the sludge to create reusable biosolids as well as alternatives to the multiple hearth incinerators for thermal destruction of sludge.
MHFs are the most common type of sludge incinerators in the USA (over 200 installations). However, many of the older MHFs could not meet new stricter air emission standards enacted in the late 1980s that took effect in the 1990s without expensive upgrades that also increased operating costs. Since then, a different design, the fluidized bed furnace (FBF), which is able to achieve the stricter emission standards has come into popular favor. The FBFs have higher combustion efficiency and lower emissions than MHFs. In a FBF, a bed of sand is fluidized by combustion air and dewatered sludge is injected into the bed where moisture evaporates and combustibles are burned at 1400-1500°F.
Exhaust gases and ash exit the top and the entrained ash is typically removed by a venturi scrubber. Because the bed of sand is fluidized (suspended in air), it must be of uniform size and density and the hydrodynamic characteristics (pressure drop) must remain constant in order that the design fluidization characteristics (bed expansion) are maintained during years of continuous operation. If the bed size/density changes (usually increasing due to particles sticking together) or the pressure drop changes due to duct clogging, then the bed will not expand as designed and incineration efficiency will suffer. Some FBFs have encountered problems with scale and/or agglomorate formation. Scale formation can impact heat transfer and partially block the flue causing high backpressures and associated system failure. Agglomerates cause bed segregation/defluidization and act like battering rams inside the reactor that can destroy the lining and other internal components. These problems have been related to sludge chemical composition (Jeffers et. al., 1999). High concentrations of sodium and/or potassium chloride salts can cause glassification of the sand bed (agglomeration). And high concentrations of iron and phosphorus can cause scale formation.
Researchers and FBF manufacturers have developed strategies for preventing these problems to allow combustion of sludges with high concentrations of these species. The prevention methods involve chemical conditioning to modify the sludge chemical matrix. There are no simple criteria in terms of the quantity of chemicals to add to a given sludge for prevention of scale and agglomeration formation. Therefore, it is necessary to determine the entire chemical composition of a sludge in order to define potential strategies for operational control of scaling and agglomeration. These strategies can then be pilot tested prior to FBF construction and/or during break-in of the completed system. The goal of this project was to determine the chemical characteristics of the SIWWTP sludge to facilitate determination of chemical conditioning strategies for a future FBF sludge incinerator.
Description and Scope of Work:
1. Coordinated with City and County of Honolulu personnel to collect duplicate representative thickened sludge samples from SIWWTP every other month for one year. This provided a total of six sets of duplicate samples to determine any seasonal sludge chemistry variations.
2. Centrifuged sludge samples in the laboratory without amendment chemicals and evaporated to dryness.
3. Conducted a proximate analysis of the dried sludge solids including: percent volatile solids (VS), percent ash (A), and high heat value (HHV) of the VS in btu/lb VS. VS and A will be by gravimetry and HHV will be by calorimetry.
4. Determined the chemical composition of the sludge ash (soluble and insoluble forms) including S, Cl, Al, Na, K, P, Ca, Fe, Mg, and Si by ICP-MS and AA methods.
5. Determined the metal content of ash including: As, Cd, Cr, Pb, Ni, Be, and Hg by the ICP-MS method.
6. Conducted an ultimate analysis of the dry sludge including: C, H, N, O, and S.
7. Brief bi-monthly reports were submitted to R.M Towill Corp. containing data collected. A final report was prepared containing all data collected and all findings.