PLTU ABOUT WASTE WATER TRAETMENT

Process Description


Unit Neutralization Pit

Regular water from power house oil sump pit, condenser pit, turbine room sump pit, boiler area sump pit, check pit 2 & 3 ( When oil is detected in the water ) will collected in the oil removal pit. ( First compartment of Unit Neutralization pit ).

The oil removal pit is provided with a belt type oil skimmer which is the one of the easiest ways of removing hydro carbon from waste water. The oil skimmer needs to be started by operator, from the PLC when oil is detected in the checkpits.

Belt oil skimmer work because of the difference in specific gravity between oil and water. Water has a specific gravity of less than one because of these differences, oil flats to the top of the water where it can be removed. A belt of oil skimmer uses oil oilophilic material ( usually made of ss or plastic ), in the form of belt to break the surface tension of the water to attract and collect the floating oil. The belt passes through a set of wiper blades via a motorized head pulley where the oil is wiped off both sides of the belt and oil is collected into oil skimmer pit. Water moves to intermediate compartment in the unit neutralization pit from the bottom of oil removal pit and then it is transferred to the third compartment by overflowing from second compartment. The third ( main ) compartment of the unit neutralization pit is provided with the air line and headers from the unit neutralization pit blower. The operation of unit neutralization pit blower will depend on the water level of water in the pit. The low level of water in pit will trip the blower.

The pH meter installed in the neutralization pit will keep a watch on the pH level of water. The acceptable range of pH in water discharged from the pit is 6.5 to 8.5. Additional caustic dosing line is provided in the UN pit. If requirement arise then the diluted caustic manual valve to the inlet line can be opened to raise the pH of the unit neutralization pit so that it is maintained within the acceptable range. This shall be a need based operation only.


Pump Operation

Two Unit Neutralization pit pumps are provided to transfer the water from the Unit Neutralization pit to waste water storage ponds 3 & 4. At high level f the water in the pt, selected pump will start and remain ON till the low level in the pit is reached.

The filling of water in WWS 3 & 4 depends on the level of water stored in them. In the beginning, the valve in the inlet line of WWS ponds 3 is open and the water is filled in the pond. This valve is be shut off on the high level of the pond and the valve in the inlet line of WWS pond 4 will open and remain open till its high level. In case of high level in both WWS ponds 3 & 4, the running UN pit pump will trip.


At high level of WWS pond 3 & 4, selected pump of respective pond will start and remain ON till the low level of that pond. In case of high level of clear water pit during system start up, the selected WWS pond pump should not start and when during operation, if the water level becomes high, then the pump should trip. In case of pH in pH control and oxidant pit going beyond acceptable range, WWS pond pump has to be tripped.


Waste Water Storage Ponds 3 & 4

The regular waste water from the Unit Neutralization pit, laboratory sump pit, administration building general waste sump pit coal analysis pit, common header from water treatment building sump pit, chlorination building sump pit and filtrate from the filter press is transfer to waste water storage ponds where it is aerated so that the concentration of solids attain a uniform value. Two no of WW storage ponds are provided in one filling and one empty condition, so as to take the benefit of continuous aeration of the water in the pond. The blowers will be OFF only in the condition of low level respective ponds. One common header is provided from the outlet of both blowers which will ensure the continuous air supply to pH control and oxidation pit.


pH Control and Oxidation Pit

Water from the WWS ponds is transferred to the first compartment and oxidation pit via the WWS pond pump. Her pH is sensed by pH analyzer. Any deviation from the acceptable range will open the valves provided in the line dosing lines to the pit.

In case of pH going below 6.5 ( set value in PLC ), the valve in the inlet of caustic dosing line will open and remain till pH attains a value of 7.5 ( set value in the PLC ). In case of pH going above 8.5 ( set value in the PLC ), the valve in the inlet of acid dosing line will open and remain till pH attains a value 7.5 ( set value in the PLC ). For the acid and caustic dosing the pH values should be fed in the set parameters at the PLC as per requirement. These values are settable and can be changed as per requirement so that the final outlet pH from the neutralization pit is between 6.5 - 8.5.

Coagulant ( Ferric Chloride ) and Coagulant aid ( Poly Electrolyte ) is dosed in the mixing compartment of the pH control and oxidation pit. This dosing only carried out when the level of pH control and oxidation pit level is healthy. The dosing rate of coagulant and coagulant aid is manually adjusted based on JAR TEST ANALYSIS and according to the water quality. The dosing of coagulant and coagulant aid should be done continuously when the plant is running. Normally 1 - 2% concentration of ferric chloride ( coagulant ) is dosed and coagulant aid of 0.05% is dosed. The dosing rate of the chemicals can be varied by adjusting the storke of respective dosing pumps. An agitator is proved in the mixing pit. The operation of agitator corresponds with the level of water in the pit. Air line from common header outlet line from WWS ponds blowers is prvided which is ensures the continuous air supply to pH contrl and oxidation pit for oxidation and help in chemical mixing.


Coagulant and Sedimentation Tank ( Clarifier )

The water from pH control and oxidation pit is transferred to the coagulation and sedimentation tank. Here the clarifier mechanism provided helps the TSS to settle down in the tank and overflowing the clear water through side wall launders to clear water pit. The sludge settled in the bottom of the clarifier is transferred to the suction line of sludge slurry transfer pumps.


Sludge Enrichment Tank ( Sludge Thickener )

The sludge from the of coagulant and sedimentation tank is transfered to sludge enrichment tank by sludge slurry pump where the Coagulant ( Ferric Chloride ) is dosed. The dosing is carried out manually by opening the valve, and only when the sludge slurry transfers pumps on. Normally the coagulant dosing valve at slude thickener is closed. Dosing of coagulant to be done only when the sludge is being transfered to the sludge thickener from the coagulation and sedimentation tank. Sludge thickener mechanism provided helps the thick sludge to settle down in the tank and overflowing through side wall launders and transfer it to WWS ponds. The sludge settled down in the bottom of the tank iis transferred to the suction of sludge enrichment transfer pumps.

This sludge is then transferred to the dehydrator ( Filter Pressure ) via two sludge enrichment tank pumps ( Screw Pumps ). The operation of sludge enrichment tank pumps is interlocked with the level switch. The pump will only start when there is sufficient level in the sludge enrichment tank and trip when the low level is actived. The pump starts after settable time for ( 2 Min ). A valve is provided for flushing the suction line of sludge enrichment tank pump. The operation of the valve is settable time based. For this section start stop facility is provided to drain the sludge and to operate the pumps as per required. To drain the sludge manually the sludge section has to be kept in manual mode and open the flushing valve and start the pump from PLC.