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| 1. Greenfield or new water treatment plants based on Tube Settler technology / clarifiers read more... |
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| 4. Augmentation or retrofit of existing plants for capacity or quality up-gradation read more... |
5. Mass application of small capacity standardized plants
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| �Reducing Water Losses in Treatment Plants� in Developing Countries Introduction |
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| Introduction : |
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Water losses in the water supply scheme range from 15% to 30% of treated water. Most of these occur in conveying and distribution system. However water treatment plants contribute to a small but significant portion of it. Water losses in a treatment plant are caused by wash-water from filters and sludge from clarifiers or settling tanks. In well designed and operated treatment plants these are up to 2% but usually can be up to 5% of treated water. It is worth taking a look at various methods adopted to reduce these losses and conserve the precious water.
In India most of the treatment plants have surface raw water as the source. Most of the water treatment plants do not have facility to reduce or eliminate wastewater discharges. The wastewater is disposed off to a water body, streams, rivers, natural drain or for land irrigation without treatment. The second and important aspect is that in government or municipal sector there are neither statutory regulations on quality and quantity of waste water discharge from treatment plants, nor it comes under the jurisdiction of pollution control boards. There are no regulations till date of these practices. In the developed and advanced countries waste water treatment and recovery is a regular feature of water treatment plants.
However in industrial sector, these discharges are treated as effluent. Adequate treatment is required to be given to wastewater to suit disposal quality standards. The main component of pollutant is suspended solids, which needs to be eliminated significantly before the discharge. Therefore many water treatment plants in industrial sector have the facility to treat the wastewater from water treatment plants. The number of treatment plants in government or municipal sector is many times more than plants in industrial sector. However it is a great paradox that the treatment is regulated only in the industrial sector.
 Counter Layout Flow Diagram
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| Re-circulation of dirty filter backwash water |
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The spent or dirty backwash water from filter beds amounts to 2% to 3% of treated water. The suspended solids content in the backwash waste is 300 to 500 mg/lit. The PH is in between 6.5 to 8. The main treatment aspect is therefore removal of suspended solids and then to recycle the wastewater back to the treatment plant.
The treatment generally comprises of gravity settling. The capacity of re-circulation tank is dependent on number of filter beds and frequency of backwashing. The re-circulation tank capacity should be adequate to hold volume of one backwash quantity for plants having less than four filter beds. For larger plants the capacity should be to store two backwash quantities. These are normally rectangular units with flat bottom. The tank is divided into two compartments to give enough residence time for gravity settling of solids and to facilitate manual sludge removal periodically. The side water depth of the tanks is observed to be between 3.0m to 4.0m and length to breadth ratio as 1:1 to 4:1.
The recycle tanks are located generally on the lowest contours of the site to facilitate gravity flow from filter beds. Plant hydraulics dictates that abnormally large freeboard is required to be provided for the tanks in many cases. The sludge removal frequency can be provided once or twice a year. The sludge is removed manually from these tanks. The consistency of the sludge can go from 10% to 15%. In the developed countries mechanical means like scrapper units are provided to remove sludge. However in Indian context it may not be feasible due to high capital, operation and maintenance cost. ,
The recycle flow rate of recovered waste water needs to be worked out carefully. It invariably involves pumps to deliver the water to Inlet Chambers of treatment units due to higher elevation. The recovered water is led up to Receiving Chamber or Flash Mixing unit of the plant. In order to avoid shock loading the rate of recycle is kept as small and uniform as possible. It is better to have more number of pumps of smaller capacity, or to provide a variable frequency drive to achieve flexibility in operation.
As an example consider a plant with eight Filter beds. The washwater generation from each Filter bed is 400 cum per wash. In this case the capacity of recirculation tank will be 2 x 400= 800 cum. Each compartment will have dimensions of 8.0m x 16.0m x 3.15 m as SWD. In order to calculate recycle rate, take the worst case of all filter beds to be washed in a single day. The quantity of water consumed will be 8 x 400 = 3200 cum. Let the time of recycle to be 22 hrs. per day. The recycle rate will be 3200 / 22 =145 cum/hr. Provide three pumps (2W + 1S) of capacity 75 cum/hr each.
The recent trend is also to disinfect the recovered water by using chlorine or other disinfectants before recycling. This is required to immobilize contaminants like Cryptosporidium or Giardia lamblia cysts.
Recirculation Tank Diagram
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| Thickening of sludge |
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The clarifier or settling tank should be a single point to take out accumulated solids (sludge) from the treatment system. The mass balance should be done in the following manner to estimate the quantity of sludge. Consider a plant having capacity of 60 mld (2500 cum/hr) and 200mg/lit as maximum suspended solids load in raw water. Assume settled water suspended solids to be 10 mg/lit. The consistency of sludge will be about 1.5% (W/V) from clarifier. The sludge generation will be {2500 x 200 = [2500 � X) x 10] + 15000 X}. Here X is the rate of flow of sludge, which turns out to be 32 cum/hr. The sludge is generated 1.5m to 2.0m below the ground level due to clarifier hydraulics. This sludge needs dewatering to remove suspended solids and to recover water (though the water recovery is not much). The sludge pit is normally well below the ground and has detention time of two to four hours. This sludge is pumped to further dewatering units. (In the above example suspended solids contributed due to coagulants are not considered).
The sludge is normally dewatered using gravity thickeners. The main objective of dewatering unit is to thicken the clarifier sludge (1% to 2% W/V) to 3% to 5%. (W/V) consistency. This is known as a thickened sludge.
The conventional gravity thickener is a circular unit like clarifier having pickets fixed to its scrapper arm. Normally they have centrally driven mechanism. The pickets are mild steel angles or flats having 1.0m to 1.5m in height. The pickets help in releasing water interlocked between the solids particles in hindered settling zone (bottom sludge). The entry of clarifier sludge to Thickener is at the center and collection of overflow is peripheral. Sometimes the poly-electrolyte is dosed at thickener inlet to improve underflow (sludge) consistency. The solids loading rate for thickener can vary from 20-50 kg/sqm/day. The hydraulic loading rate varies between 5 � 12 cum/sqm/day and detention time between 4 � 10 hrs. The side water depth is normally provided as 4.0m. The overflow from thickener normally carries suspended solids less than 100 mg/lit. This water can be again re-circulated back to the treatment plant. The sludge with lime content has better thickening ability than that of alum or ferric sludges. |
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| De-watering of sludge |
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The underflow or sludge from thickener can be further de-watered to obtain solids consistency of 20% to 30%. This sludge from de-watering units can be disposed off for land filling. The different types of units used in this process are centrifuges, filter presses or sludge drying beds.
Sludge drying beds have their use in dry and arid climate where rainfall is less or moderate. The area requirement is based on solids loading criteria of 150-400 kg/sqm/year. The drying cycle is 6 �10 days. The depth of sludge application is restricted to 300-450mm. The sand depth is 300-400 mm with Effective Size varying from 0.3 to 0.75 mm and Uniformity coefficient between 3 to 4. The sand is supported over gravel depth 200 to 300mm. Perforated laterals of cement pipes are used to convey filtrate up to central manifold system. The sludge drying beds require huge area. The performance of drying bed is also affected by climatic conditions. Sludge drying beds, though simple in operation are not very popular because of large area requirements.
On the other hand Filter presses and centrifuges are emerging to be popular choices for sludge dewatering as they are compact and require less space. The consistency of dry cake coming out of filter presses is 20 to 25%, while that from centrifuges is 15-20%. Use of polymers at the inlet further improves the cake consistency. The centrifuges normally are of solid bowl type with mild steel or cast iron as material of construction. Filter presses are plate and frame type and are designed to be operated in a batch mode.
Freezing and thawing improves the sludge consistency. However it is not very relevant technology to our Municipal sector.
In most of the developed countries regulations do not allow disposal of sludges and wastewater to water bodies or streams. The waste water from filter beds is required to be treated and disinfected before re-circulation. The recycle of sludges is getting discouraged due to apprehensions about microbial contamination. The emphasis on the process which inherently generate less amount of sludge. The sludge and waste water treatment in India and other developing countries will get much more importance in the future as it not only conserves water but also reduces environmental pollution. |
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