OPTIMISATION OF TREATMENT PLANT UNITS

OPTIMISATION OF TREATMENT PLANT UNITS

Treatment plant is an important sub work in the total system of the water supply facility. Normally plant cost does not exceed 4 to 5 % of the whole system. Conveyance system is designed for 30 years. Pumping machinery and treatment plant is designed for 15 years. Pump capacity reduces to about 12 % till the end of the design period. Hence, initially the design capacity is increased by 12 % for raw water as per MJP practices. Further hours of pumping are restricted to 20 hours at the end of design period. Hence, pumping rates are increased by 20%. With all this parameters rated plant capacity is increased. As other components are designed for 30 years the location of second stage treatment facility is practically decided. Modifications in treatment plant capacity to slightly on higher side does not increase the system cost much but it ensures continued treatment after decision is taken to increase the rate of pumping or adding new source. Upgrading treatment facility is normally found to be delayed. It is routed through various procedural formalities and sanctions from authorities. Quantity problem has an upper hand on the quality requirements. Hence, it is always better to have safe margins for the treatment facility and it can be stretched to optimization of the existing utility. At Yavatmal pumping was doubled and existing plant was tuned or rest to the new flow conditions locally and it was in use for about 5 years till the new plant was constructed. New unconventional treatment plants constructed Pandharkawada and other 6 places has an intrinsic capacity to deal with double flow with very minor modifications. Such aspects are introduced in the designs keeping the system maintenance orientation. Amaravati system of augmentation had the sanctioned capacity of treatment plant as 69 MLD, however, using different factors actual construction was 95 MLD. It is to mention that construction cost accepted was within the sanctioned provisions.

1. INTRODUCTION
1.1 Plant capacity is measured on the basis of the flow of water through treatment units per hour. It can be represented by flow per day, by converting flow per hour to flow per day. Some units of the plant are designed for even double the flow but in general all the units are designed for 20% over loading. Normally there is an apprehension that the filters are required to design for 20 % overloading. This is practically not correct. If flow is increased all units are hydraulically loaded. Filters are more loaded during sequential washing of beds. With the increase in loading there are no physical cost incidence, except relevant to cost on MLD basis. Plant capacity in other terms can be treated as the hydraulic capacity. Though on way the water is purified while flowing through the plant, the incoming water quality is a subsidiary parameter to decide the water treatment plant capacity.
1.2 Water enters the Treatment Plant at higher level and leaves the plant at lower level. The difference of the level is normally of the order of 6 m. to 8 m. Hydraulics of each unit is different, but water looses its head through the treatment plant and gets purified.
2. PRESENT DESIGN PRACTICES FOR PRE-SEDIMENTATION UNITS
2.1 The design of each unit is based upon the hydraulics of water flowing through that unit. It may be associated with other parameters related to the physical and chemical impurities in the water. Aeration fountain and measuring channel both have no moving parts. They can be designed for higher flow and hence it is a practice to provide these two units for the maximum flow or double the design flow.
2.2 Flash mixer is designed for the detention period of 1 min. and if this unit is operated for double the flow, then the detention period is reduced. Still, the size of this unit is comparatively very small and hence civil construction cost is less. Initially this unit may be constructed for double the flow. Otherwise flow through this unit may be diverted through distribution chamber so that flow run time is increased and the same unit can be used for more than the designed flow. Regarding the mechanical equipment of flash mixing the motor H.P. ranges from 5 H.P. to 8 H.P. and hence, providing some more initial H.P. will take care of the increased flow that it may be required to handle. Care should be taken to isolate the F.S.L. in the flash mixer by distinct level difference from measuring channel sill. However, this is dependent on the down take flow from flash mixer to flocculater. This pipe line should be capable of taking maximum possible flow or its hydraulics should be decided by designing the line for double the flow. Further as these lines are to be laid beneath the Clariflocculator regular standby line should be provided. Initially, valve controls shall have to be operated but this arrangement can take care of double the flow and economics is not affected much. Initial construction of tapered flocculation has better overloading capacity.


3. IMPORTANT ASPECTS OF DESIGN OF CLARIFLOCCULATOR UNITS:-
3.1 Intake column in the flocculator is important component for design. The main aspect is that the velocity flowing upwards should be such that there should be no settlement, and hence the diameter should be minimum. If the same column is used for double the flow upward velocity is increased. Hence, this column should be designed in such a way that in case of design flow the velocity should be such that the particles should not settle and in case of double the flow the velocity in the column should not be more than 1.8m/Sec. Hence, the velocity range should be 0.8m/Sec. to 1.8m/Sec. during design flow and double the design flow. It is economical to consider the size of intake column of the optimum diameter; considering the possibility of overloading.
3.2 Flocculator detention time should be considered as 30 min. of flow/hr. Design flow may be1.2 times the normal flow. If the depth and diameter is designed according to this design flow, the flocculator capacity can be calculated. Detention time and side water depth are the only parameters in design of flocculator unit. When double the flow is passed through this unit, detention time is reduced. Hence, initially slightly more detention time may be provided. Civil construction cost will not be increased disproportionately. By the application of accurate dosing of chemicals, the floc formation can be satisfactorily achieved. Flocculator mechanism shall be provided with sufficient paddles, so that the slow mixing by paddles can be effectively achieved. Dorr-oliver type mechanism gives more satisfactory results. Using the same units and by changing the speed of rotation slightly, required ‘G’ valve can be achieved while operating on double the flow conditions. The question of optimization of ‘GT’ valve parameter can be solved by increasing slightly the initial detention time of flocculator.
3.3 Clarifier is the most important unit as far as handling double the normal flow is concerned. Clarifier area is calculated on the basis of assumed over flow rate. Approximately, 1.5m/hr. is commonly used overflow rate. This overflow rate is for assumed design particle which should settle in that clarifier within given detention time. It means that, if the flow is doubled, than design particle should not settle in the tank and it will be carried away in the further units. However, this is further depended upon the weir flow rate. If the weir flow rate is as small as possible say 200 cum/m/day; then the tendency of the design particle being carried away in the further units is slightly curbed.
3.4 In view of the future planning, hence, the clarifier is most important unit to be carefully designed. Even by rearranging the capacity or detention time of the clarifier and restricting the side water depth, the area of the clarifier can be increased. Without increasing the civil construction cost much more. If initially larger diameter clarifier is provided then it is possible to use the same clarifier for double the flow. For bigger plants, optimum capacity of CLF shall be 30 MLD. Increasing number of units is more reasonable than to have option of larger size units.
3.5 Though theoretically, the overflow rate indicates the design velocity of the particle to settle in the clarifier. Particle settling velocity will be decedent on the temperature and other climatic conditions.
3.6 Designing for the higher flow will increase the diameter of this unit, but it must be pointed out that most of the designers are inclined to reduce the diameter to save in construction cost and still they may try to satisfy the conditions of overflow rate as well as far the detention time. Increasing the depth and restricting the diameter will give the same detention time but for clarifier depth is not important and the diameter is important as far as the overflow rate is concerned. Further, the increased diameter will also provide for the increased weir length. Even a two edged peripheral weir will not cost much but the commercial designers do not adopt to it for saving the initial cost. This tendency should be specifically curbed by providing specific conditions in the tender.
3.7 In the design of clariflocculator, Turbidity of incoming water is not a parameter. It only deals with the average size of the particle of turbidity which should settle in the clarifier. Turbidity of incoming water has large variations. In the rainy days, it may be up to 4000 ppm and in other days it may be reduced to 150 ppm. Theoretically, sedimentation should remove all the turbidity, but working design permits outlet turbidity from clariflocculator as 15 ppm. In terms of efficiency in rainy season this unit has the removal efficiency 4000 – 15 = 3985 = 99.62%
4000 4000
Whereas in other days the turbidity removal efficiency may be of the order of
150 – 15 = 135 = 90%
150 150
The increase in efficiency in rainy season is normally attributed to Alum dosing and floc formation. With the increase in hydraulic loading, Turbidity to be handled is also increased. It will require higher Alum dosing and more effective flocculation by adjusting ‘G” valve. If the same diameter floc is formed, theoretically it should not settle in the clarifier as design overflow rate will not permit to do so.
Actual overflow rate is much more; and hence upward velocity is much more and theoretically all the flocs should be washed away to the further units.
3.8 Overflow rate is related to the size of floc. In the flocculation zone it is assumed that the floc formation shall be of same size. But floc formation will depend upon the incoming turbidity and the Alum dosing, effective mixing of Alum dose, and impurities in the Alum. It will also depend upon the entry of water in the flocculation tank and velocity of entry of the water. Size of floc will also depend upon the effective paddle area and paddle speed. Even with the control of all such parameters it can be found that size of floc does not remain constant. Hence, the floc sizes will have a gradation. The floc, size of which is higher than the particle size for which overflow rate is decided, will settle in the clarifier.
3.9 The particle size which is not designed to settle in the tank, with the same overflow rate will try to rise in the tank. It is possible that its position in the tank be balanced up to the weir level. In that case also the particle will not be carried away. Hence, the overflow rate of 1.5 m /hr. will not indicate that particular size of particle will settle only, but it may also indicate that some lighter particles will take more time to settle but will remain in suspension. It is observed at Yavatmal Treatment Plant, that even after doubling the flow, the outlet turbidity was possible to be controlled within 20 ppm range. Such overloading of the plant is going on at Amravati Treatment Plant and it is experienced that outlet turbidity can be controlled in the same clariflocculator.
3.10 Controlling the weir flow rate and maintaining the clear overfall conditions in the launder is most important consideration. Hydraulically, the channels should be capable of accommodating higher flows and it is very important, as it will not overload the units. Economics is not disturbed by providing wider channels. Only thing that, it is required to foresee the position, some years ahead when flow is required to be increased. Instead of going for separate treatment unit, it is required to exploit the provisions existing. However, such exploitation can be anticipated and at least hydraulics in that stage can be imagined and provided for, at the present stage without disturbing the economics.
3.11 Standard design of Treatment Plant was published in the Indian Water Works Journals. It provides for a detention time of clairflocculator as 3 hours, standing water depth as 3.5m. and additional sludge storage capacity of 25% of detention time. Similarly flocculator detention time is taken as 30 minutes as normal.
3.12 However, the commercial designs try to economics in the diameter of clarifier by increasing the S.W.D suitably for the design conditions of flow. The clariflocculator does work, but its utilization for the higher flow becomes limited. Hence, it is to point out that the real economics does not apply to the utilization in the present stage, but it is advantageous to continue utilization in the future stage. If this concept is acceptable the hydraulic design of various channels and piping for the higher flow (suitably double the flow) needs no justification.
4. FLEXIBILITY OF OPERATION IN FILTERATION UNITS:
4.1 The filters are designed for filtration rate of 5000 lit/sq.m./hr. This hydraulic loading will take care of incoming turbidity of 20 ppm. and the filtrate will be available having turbidity below 1 ppm. The flexibility in operation of filters, is due to following reasons,

a) The nature of sand can be suitably decided. The parameters, uniformity coefficient and effective size for the sand can be varied to suit the rate of filtration. The sand parameters can be changed even during the course of maintenance.
b) The period of backwashing can be adjusted so that the filters are prevented from choking and cracking. After a certain predefined loss of head through filters, the backwashing can be done and the filters are again ready to receive a fresh load. Normally filters are designed for 20% hydraulic overloading. During backwashing of one of the units, others are overloaded hydraulically.
c) Filtrate quality, if not attended to properly, gets disturbed, say up to 2 to 4 ppm from the normal expected quality below 1 ppm. This change however, is not noticeable. Hence, the delay in attending to properly, though not justified, remains in tolerance limits.
4.2 Backwash water is wastage. as far as the treatment plant is concerned. Parameters indicating minimum wastage of water are available. Normal wastage is 2%, based on annual average flow through treatment plant. With design loadings, the filter beds are not choked within 24 hours filter runs. If the loading is increased the backwashing becomes necessary, earlier. The wastage of backwash water is then increased in that preparation. If such wastage is arranged to be re-circulated, the parameter relating to wastage due to backwash water; is taken care of.
5. ADVANTAGES OF RECIRCULATION OF BACKWASH WATER
5.1 Recirculation of backwash water reduces the final wastage of water. It adds to the turbidity of incoming water. It improves flocculent settling. If the clarifier wastage is also re-circulated then it adds, thicker turbidity in the incoming water and settling characteristics are improved. Alum is also partially re-circulated hence dose of Alum required is reduced.
5.2 On comparison it can be revealed that recirculation of waste water is finally cheaper.
6. CONCLUSIONS;
6.1 In most of the Urban Water Supply Schemes design demands for the scheme as a whole do not match with the actual demands of the city due to fast urbanization. The augmentation stage comes earlier than anticipated. Hence, the city remains in difficulties as far as water supply is concerned. It is due to this reason the designer should plan for the next stage also. The treatment units should not be designed only for the present stage; but they should be designed for higher stage. Economics should not be considered for the initial stage but considering its use in the later stage.
6.2 In case of existing treatment facilities overloading should be tried preferentially though in some cases the plants are overloaded in the pre-augmentation stage. Necessary changes can be thought of in the various units. If it is obligatory to add facility, only sedimentation facility should be tried to be added in the first instance.
6.3 In all the cases, channels, overflows, by passes and other pipelines should invariably and economically designed and provided to carry double the design flow. Economy in restricting the total head loss through treatment plant should better be avoided.
6.4 A second thought before construction of a new treatment plant will be helpful considering the optimum utilization of existing facility.