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Wastewater Treatment & Recycle An Integrated Approach
Ajay Jindal, Chief Manager, Water Technologies, Larsen & Toubro Limited and - Shama Kamat, Water Executive, Engg.& Const. Division, Larsen & Toubro Limited 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.

Following primary treatment, permanent hardness is removed by soda ash addition and second stage clarification. The treated effluent is then processed in a seeded slurry brine concentrator unit to recover high quality distillate. The residual brine from this unit is further concentrated in a forced circulation crystallizer unit to recover balance water as distillate leaving concentrated brine containing mix salt crystals that is dewatered in a filter press to separate out dry salt cake ready for disposal as solid waste. The recovered distillate from the thermal treatment system is recycled as make-up to cooling tower or demineralizer system.

Aquatech has executed and commissioned five such plants treating FGD purge stream for recycle for ENEL, Italy the 2nd largest utility power company in Europe. The picture below shows one of the sites with this type of equipment. All the five plants are in operation for over three years now.

Refinery and Petrochemicals Wastewater Treatment for Recycle
Refinery and Petrochemicals wastewaters are characterized by the presence of high levels of oil and grease, soluble organics and dissolved solids. The possible treatment options are:

1. Oil and grease removal using conventional and enhanced gravity separation equipment followed by conventional biological treatment using either fixed film and/or suspended activated sludge bioreactors. The biologically treated effluent is then polished using multi-media filter and granular activated carbon filter to remove suspended solids and residual organics. Even though, the treated effluent has passed through such exhaustive treatment, yet it is rarely found to be suitable for recycle. The primary reasons for not being able to recycle this effluent are residual level of organics, oil and grease, suspended solids and total dissolved solids.
2. The treated effluent obtained through the treatment train described above needs additional clarification and filtration for suspended solids removal, which can be fine sand media filtration or membrane filtration e.g. Ultrafiltration. Following filtration, conventional reverse osmosis at considerably lower recovery (less than 75%) or High Efficiency Reverse Osmosis (HEROTM) at much higher recovery (greater than 90%) can be implemented.
3. A relatively newer option for this type of wastewater being pursued by the industry is membrane bioreactor (MBR). Following free and emulsified oil removal, the conventional activated sludge process is replaced by submerged or non-submerged membrane bioreactor which combines advanced biological process with membrane filtration resulting in much superior treated effluent quality both for suspended solids and residual organics (Typically Turbidity < 0.2 NTU and BOD5 < 5 mg/l). The treated effluent can either be recycled for irrigation or cooling tower make-up if permitted with respect to TDS or processed through conventional reverse osmosis or high efficiency reverse osmosis (HEROTM) for further reduction of soluble impurities including TDS.

Reliance Industries (RIL) has implemented multiple wastewater recycle treatment systems utilizing HEROTM at Jamnagar, one of the largest refineries in the world, for their different phases of refinery expansion projects. The overall recovery for these plants is in the range 85 to 90 per cent. The recovered permeate is recycled for irrigation within the plant area and as partial make-up to the circulating cooling water system.

A pilot test was recently carried out to treat a petrochemical plant’s wastewater in Middle-East using a combination of Aquatech’s Enhanced Membrane bioreactor (Aqua-EMBR) and HeroTM technology to recover 90 per cent of effluent that could be used as make-up to the demineraliser system for the petrochemical plant. The pilot was successful in meeting the quality criteria set by the customer. The following table shows the quality of feed wastewater, treated effluent quality ex-Aqua-EMBR, permeate quality from HeroTM and the quality requirement set forth by customer for the feed to the demineraliser system.

The picture ( fig no 4 )shows the pilot unit that was employed at the petrochemical plant site to demonstrate the configuration of Aqua-EMBR and HEROTM.

The advantages of Aqua-EMBR over submerged MBR systems are as enumerated below.
1. Aqua-EMBR system (membrane modules) has no membrane tank, it can be built much quicker with less risks for contractors
2. Aqua-EMBR system offers an operator friendly working environment as opposed to obnoxious environment in case of submerged systems.
3. The flux is ~50% higher which equates to 50% less surface area of membrane needed per unit volume permeate production. This results in:

• Electrical power consumption is 10% to 15% lower than submerged systems due to use of airlift pump effect.
• Aqua-EMBR has the tightest membrane pore size resulting in highest effluent quality, which is an important factor for re-use purposes and future regulations.

Domestic Sewage Treatment for Recycle
Aquatech has a long and successful track record of providing systems for recycling secondary treated sewage. Such systems have utilized conventional, staged media filtration or membrane filtration followed by conventional reverse osmosis or HeoTM. The high quality permeate obtained in the process has been used either as cooling tower make-up or boiler feed make-up.

Aquatech has recently been awarded a sewage treatment and recycle project for Mumbai International Airport, India. The turn-key scope of work includes complete treatment from receipt of raw sewage to delivery of high quality, low TDS RO permeate that will be recycled as make-up for the circulating cooling water system for the airport facility. The design capacity of the system is 10 MLD.

The primary treatment consists of coarse and fine bar screens followed by a grit chamber. The secondary treatment comprises an advanced and State of The Art biological treatment technology known as Cyclic Activated Sludge System (CASSTM) followed by disinfection. There are two CASSTM basins to handle the design flow. The disinfected effluent is then polished in media filters for removal of fine suspended solids. Part of the treated effluent is further processed in three trains (two working + one standby) of ultrafiltration units followed by two trains (one working + one standby) of conventional reverse osmosis units. The RO capacity is 3.2 MLD permeate at 75 per cent recovery. The project is currently under construction. The following section briefly describes the CASSTM process and its advantages over conventional activated sludge process.

Cyclic Activated Sludge System (CASSTM)
Cyclic Activated Sludge System (CASSTM) as the name suggests is one of the most popular sequencing batch reactor (SBR) processes employed to treat municipal wastewater and wastewater from a variety of industries including refineries and petrochemical plants. Aquatech has an agreement with AECOM (erstwhile Earth Tech), UK, the licensor of this technology to supply CASS™ technology in India on exclusive basis to both municipal and industrial markets.

It offers several operational and performance advantages over conventional activated sludge process. The CASS™ SBR process performs all the functions of a conventional activated sludge plant (biological removal of pollutants, solids/liquid separation and treated effluent removal) by using a single variable volume basin in an alternating mode of operation, thereby dispensing with the need for final clarifiers and high return activated sludge pumping capacity.

The Cyclic Activated Sludge System (CASSTM) incorporates a high level of process sophistication in a configuration which is cost and space effective and offers a methodology that has operational simplicity, flexibility and reliability not available in conventionally configured activated sludge systems. Its unique design provides an effective means for the control of filamentous sludge bulking, a common problem with conventional processes and other activated sludge systems.

The essential features of the this technology are the plug-flow initial reaction conditions and completemix reactor basin. The CASSTM SBR reactor basin is divided by baffle walls into three sections (Zone 1: Selector, Zone 2: Secondary Aeration, Zone 3: Main Aeration). Sludge biomass is intermittently recycled from Zone 3 to the Zone 1 to remove the readily degradable soluble substrate and favor the growth of the floc-forming microorganisms. System design is such that the sludge return rate causes an approximate daily cycling of biomass in the main aeration zone through the selector zone. No special mixing equipment or formal anoxic mixing sequences are required to meet the effluent discharge objectives. The basin configuration and mode of operation enables combined nitrogen and phosphorous removal mechanisms to take place through simple "one-shot" control of the aeration.

CASSTM utilizes a simple repeated time-based sequence which incorporates:

FILL - AERATION (for biological reactions)

DECANT (to remove treated effluent)

Advantages of CASSTM
The CASSTM SBR maximizes operational simplicity, reliability and flexibility
Important reasons for the use of the CASSTM SBR over the conventional constant volume activated sludge aeration and clarifier process include:
• The ability to operate under continuous reduced loading through simple cycle adjustment.
• The ability to operate with feed-starve selectivity, So/Xo operation (control of limiting substrate to micro - organism ratio), and aeration intensity to prevent filamentous sludge bulking and to ensure endogenous respiration (removal of all available substrate), nitrification and denitrification together with enhanced biological phosphorus removal.
• The ability to operate for simultaneous (co- current) nitrification and denitrification by variation of aeration intensity.
• The ability to tolerate shock load caused by organic and hydraulic load variability. The system is easily configured and operated to adjust for short-term diurnal and long-term seasonal variations.
• Elimination of secondary clarifier.
• Elimination of separate load equalization. The CASSTM SBR basin is in itself an equalisation basin and a clarifier with a much lower solids flux when compared to conventional clarifier design.
• An inherent ability to remove nutrients without chemical addition through control of oxygen demand-oxygen supply operation.
• Provision for energy optimization through nutrient removal mechanisms. The feed water carbonaceous BOD used in denitrification and enhanced biological phosphorus removal reduces overall oxygen demand and hence energy requirement.
• Capital and operating cost advantages.
• Minimum footprint and reduced land requirement.
• Provision for easy plant expansion through simple modular and common wall construction.

All the examples illustrate that one needs a diverse range of technology portfolio to solve a given wastewater treatment application especially when the objective is to recycle. A holistic view must be taken for each individual project and application to ensure the correct techno–economical solution. Aquatech with the help of a wide technology arsenal has been uniquely positioned to solve complex waste water problems and maximize recycle and reuse.