TDS and Sludge Generation Impacts from Use of Chemicals in Wastewater Treatment

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
The paper analyses theoretically expected TDS changes resulting from use of typical chemicals and from biological treatment. This is compared with the experimental data from full-scale facilities and from bench tests. Sludge generation effects are also discussed with supporting data.

INTRODUCTION
With progressively more stringent limits and expanding list of controlled parameters wastewater treatment plants increase their dependency on treatment chemicals. Chemicals are used to precipitate phosphorus and heavy metals, to improve flocculation/settling, for disinfection and for pH control, among others. At the same time, in some states (such as New Jersey) regulators are beginning to enforce effluent TDS limits based on the 500 mg/L in-stream freshwater quality criterion. For plants with limited dilution in the receiving stream, use of chemicals places additional strain on the ability of the plant to meet its TDS limit. In some other systems, typically industrial, which rely on reused water, use of chemicals contributes to build-up of the inert dissolved solids.

The paper analyses theoretically expected TDS changes resulting from use of typical chemicals and from biological treatment. This is compared with the experimental data from full-scale facilities and from bench tests. Sludge generation effects are also discussed with supporting data.

THEORETICAL EFFECT OF CHEMICALS ON TDS
The expected impact on TDS of addition of various chemicals is derived from underlying stoichiometry. These effects range from straightforward, as is in the case of chlorination/dechlorination chemicals and neutralization with caustic or acid, to more complex, when the effect of alum or ferric salts addition on TDS depends upon the stoichiometric excess ratio in relation to orthophosphates. Similarly, use of lime or bicarbonate could have varied impacts depending upon alkalinity, hardness and pH. The impact of alum, ferric salts and chlorination - dechlorination chemicals will be also complicated by a desired neutralization level. The conversion factors developed for most common chemical addition cases are summarized in Table 1.

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