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System Overview

1. Internal Collection System (ICS) and Conveyance Network:

The 680 member units of the estate are spread in an area of 13.5 sq. km. in Vatva industrial estate. The effluent from each member unit is conveyed through ICS to CETP in a systematic manner. Adequate arrangement is made for monitoring quality and quantity of effluent from each unit. The details of the ICS are described briefly below

M/s Dalal Consultants and Engineers Ltd., Ahmedabad provided consultancy services and detailed engineering for the establishment of Internal Collection System and Conveyance Network. The 680 member units which are scattered in different parts of the complex are covered by 92 Sump Rooms from where the wastewater flows by gravity to 6 Pumping Stations. The effluent is pumped from these Pumping Stations into CETP for treatment.

All the member units discharge their effluent from their overhead discharge tank into respective sumps. The butterfly valves and magnetic flow meters are provided in each sump room for measuring flow rate of effluent from each member unit. It is obligatory on the part of every member to install overhead effluent collection tank having enough holding capacity to facilitate gravity flow to the respective sump.

The gravity mains of the ICS have a total pipeline length of approximately 18000 meters; with sizes varying from 250 mm to 600 mm diameter. These pipelines are made of RCC and stoneware. The rising mains are made of HDPE having a total length of approximately 6200 meters with sizes varying from 180 mm to 400 mm diameter.

2. Treatment Process

Each member unit is required to arrange for pretreatment of the effluent to meet the CETP inlet norms.

The CETP consists of the following units.

  • Equalization Tank
  • Flash Mixer
  • Flocculator
  • Dissolved Air Floatation Unit
  • Aeration Tank
  • Secondary Clarifiers

The pretreated effluent from the member units is pumped to the Equalization Tank. The Equalization tank is designed to provide residence time of 24 hours under the maximum flow condition. The contents of the Equalization Tank are thoroughly mixed / equalized with the help of coarse bubble diffused aeration supplemented by a Turbo Aerator.

From the Equalization Tank the wastewater flows through flow-control arrangement into Flash Mixer where coagulants and flocculants are added. After complete mixing of the chemicals with the effluent it flows into Flocculator where coagulation and flocculation of suspended solids, colloids and some of the dissolved pollutants take place. The overflow from the Flocculator goes to Dissolved Air Floatation (DAF) Unit. The unit works on the principle of super saturation of the liquid with dissolved air. The super saturation is achieved by mixing pressurized recycled wastewater with compressed air. The saturated recycle flow is released at about atmospheric pressure in the DAF Tank which results in the formation of fine bubbles. These fine bubbles get attached with flocculated particles and are floated to the top of the tank. These particles as well as those settled at the bottom of the tank are removed and collected by the top and bottom scrappers in the form of primary sludge and is collected in the Primary Sludge Holding Tank from where it is sent to Centrifuge Decanter for dewatering. The dewatered sludge is then disposed off into Secured Landfill Site at Vinzol.

In the outlet of DAF the concentration of suspended solids is maintained at less than 100 mg/l. Some reduction in COD content is also observed due to removal of organics in colloidal state. This effluent flows to Aeration Tank by gravity. The total capacity of Aeration Basin is 27800 cu. m. Medium bubble aeration supplemented by 18 nos. Triton type Aerators supply air to maintain the desired DO level and also to maintain the MLSS in suspension. The organic matter is aerobically degraded by the micro-organisms contained in the activated sludge. Nutrients in the form of nitrogen and phosphorous containing compounds are added as per the requirement. The various parameters including MLVSS, MLSS, F/M ratio, DO level and concentration of nutrients are closely monitored and maintained to achieve the maximum degradation of organics.

The effluent containing MLSS flows into the two Secondary Clarifiers after passing through the Flow Distribution Chamber. Provision is made for adding coagulant in the Clarifiers depending on the requirement. The suspended solids get settled by gravity and are collected at the bottom of the Clarifiers from where they are taken to the Sludge Sump. This is activated sludge containing biomass along with inert. A portion of this activated sludge is recycled into the Aeration Tank for the maintaining biomass in the form of MLVSS at the desired level. The rest of the sludge is taken into Sludge Holding Tank from where it is pumped to Centrifuge Decanter for dewatering. The dewatered sludge is transported to SLF for final disposal.

The supernatant from the Clarifiers is collected in Holding Tank / Pumping Station from where it is pumped to the Mega Pipeline for ultimate conveyance into river Sabarmati at Pirana after mixing with the treated sewage discharged from Pirana STP of Ahmedabad Municipal Corporation.

The Process flow diagram: Annexure – D

3. Monitoring and Laboratory Facilities

In order to assess and control the quality of effluent from the member units elaborate monitoring system is practiced.  Accordingly sampling of effluent of each member industries is carried out daily during the period of discharge and the quality is determined in terms of pH and TOC of all effluent samples. Additionally depending on the raw materials and products other parameters like ammonical nitrogen, phenolic compounds and heavy metals are also determined. The quantity of the effluent being discharged by the member units is also measured individually with the help of magnetic flow meters. The member units are intimated of the results and are also charged on the basis of the quantity and quality of the effluent.

The performance of CETP is assessed by determining the following parameters…

  • Primary Treatment Stage: The equalized raw effluent and outlet of DAF unit are analyzed for pH, TOC, BOD, COD, TSS, Oil and Grease, TDS, Ammonical Nitrogen, Phenolic Compounds and Heavy Metals at the frequency of once per shift.
  • Secondary Treatment Stage: The contents of the Aeration Tank are analyzed to determine pH, DO, DOUR, MLVSS, MLSS, Nutrients and SVI. The overflows of the Clarifiers is analyzed to determine pH, TOC, BOD, COD, TSS, Oil and Grease, TDS, Ammonical Nitrogen, Phenolic Compounds and Heavy Metals to ensure that the standards specified by GPCB are complied with.

Laboratory Facilities:

At CETP Site a well equipped Laboratory and associated facilities have been established. These include the following sections…

  • Physico-Chemical Lab.
  • Microbiology Lab.
  • Instrumental Analysis Lab.
  • R&D Section.
  • Sample Preservation and Storage Section.