Wastewater Treatment & Recycle

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Courtesy of Aquatech International Corporation

Fresh water is a scarce commodity; this fact is well acknowledged by one and all. People at all levels be it general public, governmental bodies, and industries understand the implications of good quality water availability in terms of quantity, cost and wastage. As a consequence, there is a growing consensus on wastewater treatment for recycle/reuse. This paper briefly describes the treatment configurations implemented for select wastewater treatment applications by Aquatech.

To achieve wastewater treatment for recycle or zero liquid discharge (ZLD) on an economical basis, it is not one technology that fits all, but one needs multiple technologies and careful integration of these. Each wastewater has its own nuances and to deal with these requires intimate knowledge of water chemistry and experience in the applicability of each technology. Besides technical feasibility, economic feasibility of a given treatment process for recycle is very critical and advancement in membrane processes has aided towards this cause.

Aquatech has successfully implemented integrated solutions for the treatment, recycle, and ZLD of various challenging waste streams which include municipal sewage, cooling tower blow down, flue gas desulfurization scrubber's purge stream from high sulfur based coal fired power plants, produced water from oil and gas production industry and effluent recycle for petroleum refineries, petrochemical and chemical plants etc.

Produced Water Treatment for Recycle
Produced water from an oil field that extracts oil by steam flooding is contaminated with oil, high dissolved solids including calcium, magnesium, chlorides, sulfates and silica. To recycle this type of wastewater as make-up to a once-through steam generating boiler, the possible treatment approaches are:

1. Oil and grease removal using
conventional and enhanced gravity separation equipment followed by lime - soda ash and mag-oxide clarification followed by ion-exchange softening. The ion-exchange softening in this case is generally weak acid cation (sodium form) exchanger.
2. Oil and grease removal using conventional and enhanced gravity separation equipment followed by ion-exchange softening. The ion-exchange softening in this case can either be a combination strong acid cation (sodium cycle) exchanger followed by strong acid cation exchanger (sodium cycle) or strong acid cation (sodium cycle; exchanger followed by weak acid cation (sodium form) exchanger.
3. Oil and grease removal using conventional and enhanced gravity separation equipment followed by mechanical vapor compression (MVC) evaporator. Depending on the calcium hardness, sulfate and silica levels in the feed water, MVC unit can operate in either seeded slurry or non-seeded mode.

Depending on the feed water characteristics, the cost of raw water availability and disposal options of wastewater, one of the already mentioned treatment options can be selected. Aquatech has implemented one of the world's largest produced water recycle systems based on the last treatment option mentioned above. The project, which was for a major western oil company for a site in the Middle East, was executed on a complete EPC basis. This plant consists of seven trains of MVC driven twin evaporator units, which can be operated in seeded and non-seeded modes of operation depending upon the produced water quality. The plant has been successfully operating for over two years.

Based upon the results and performance on Phase one of the project, the client awarded the next phase of their steam flood Enhanced Oil Recovery (EOR) project beginning last year. The selected treatment scheme in this phase is based on the second option of ion-exchange softening listed above. This phase is currently under construction and slated for commissioning later this year.

Flue Gas Desulfurisation (FGD) Scrubber Purge Stream Treatment for Recycle
An FGD scrubber's purge stream is very high in total dissolved solids and sparingly soluble heavy metal ions including very high calcium and magnesium, the main hardness causing ions. This wastewater has a high scaling tendency and is very difficult to treat for disposal leave alone recycle. The possible treatment approach for recycle is as below.

Primary treatment includes removal of suspended solids, supersaturated calcium sulfate and sparingly soluble heavy metals precipitation by addition of coagulant, coagulant aid, and lime through clarification.

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