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CHALLENGES DUE TO WASTEWATER CHARACTERISTICS AND DISCHARGE LIMITS
Industrial wastewaters (WWs) from refineries and petrochemical and chemical sites have several common features with municipal wastewater in terms of organic pollution that can be removed in a final centralized, biological process.
Often the biodegradability of the ‘chemical oxygen demand (COD)’ in industrial WWs can be less than that of the municipal wastewater or the initial concentration higher. The discharge limits for the treated effluent are often the same as for municipal wastewater, leading to a need for higher removal efficiency, however.
As for municipal wastewater treatment systems, the biological processes need to be installed downstream of the primary treatment. The main difference is that, for municipal wastewater, primary treatment removes only inert materials and grit, which are common to all inflows, while, typical pre-treatments for industrial WWs are:
- dedicated to specific streams that are then combined into the final, common, biological polishing unit;
- based on various unitary processes specific to certain streams and their key pollutants - e.g. oil removal; metal precipitation; H2S, ammonia or VOC stripping, etc. In general, industrial wastewater treatment requires a ‘system’, not a ‘plant’, consisting of multiple units dedicated to one or more influent streams.
- designed for equalization, so that pollutant flows and mass loads are balanced, enabling the final, central, biological unit to operate as a process reactor, fed with known proportions and amounts of different streams, thus enabling the final quality of the treated water to be guaranteed. Equalization, like pre-treatment, must be provided as a system, incorporating multiple units dedicated to different wastewater streams.
One of the main challenges concerns industrial wastewater composition, which affects the process, construction material selection, and the potential for re-use, perhaps because of salinity or total dissolved solids (TDS). TDS, especially chloride, sulfate and sodium, if added in the industrial production process, are not removed by conventional treatment (only desalination is effective). Sometimes they can cause issues in a biological treatment plant:
- adverse impact on biomass flocculation, perhaps leading to solid-liquid separation issues in the clarifier, especially if the salinity concentration changes frequently;
- above certain TDS thresholds, slow nitrification kinetics and potential process instability arise, as well as impacts on sludge growth; TDS concentration can also cause issues...