RECIRCULATOIN OF WASH WATER
During the daily operations of treatment plant washing of filters is a regular activity. For washing of filters pure water is used and normally it is wasted. In case of higher capacity treatment plants the quantity of wash water wasted is substantial 3% to 5% of the capacity of treatment plant is estimated as waste.
Wash water quantity waste can be minimized by recirculating the waste water by creating an appropriate storage. In case of new treatment plants recirculation can be one of the regular unit and can be simultaneously constructed. However, in case of old treatment plant recirculation feature can be added during maintenance.
Sample calculations are given as below so as to arrive at the economics.
RATE OF FILTRATION 5M3/M2/HR
BED AREA 100M2
MAXIMUM FLOW 12000M3 /DAY
RATE OF BACKWASH 600LIT/M2/MIN
FOR 10 MIN BACKWASH FLOW 600*10*100LIT/DAY 600 M3/DAY
% FLOW OF BACKWASH 5%
ASSUME 80% BACKWASH FOR RECIRCULATION 4% 480 M3/DAY
COST OF RECIRCULATION WATER @ RS. 7.30/M3 480*7.30 = 3504 RS/DAY
REVENUE FROM RECIRCULATION WATER / YEAR 1278960 RS/YEAR
% RECVERY @ 80% OF ABOVE 1023168 RS/YEAR
MINIMUM STORAGE OF RECIRCULATED WATER 480 M3
RATE OF PUMPING 480/24 M3/HR 20 M3/HR
HEAD 15 M
HP 10000*15*1.2/75*3600
ASSUME 5 HP
COST OF CONSTRUCTION OF SUMP 480 M3 480*3000 1440000 RS
COST OF PUMPING MACHINERY FOR 5 HP 5*5000*2
=50000 RS
ENERGY CHARGES PER YEAR @ RS.5/UNIT 5*24*365*5 RS
=219000
OTHER CHARGES 200000 RS
COST OF PUMP HOUSE FOR 16 SQ. M 16*2500 =40000 RS
TOTAL COST 1749000 RS
Hence it can be seen that capital expenditure incurred stands recoverable within very short period. It can be taken up during the maintenance of the scheme.
Even if the capital cast is taken as the loan, the total repayment is expected within 3 to 3.5 years. However, for capacities more than 25 mld plants recirculation will be more justifiable, but it can be thought of for lesser capacities where there is a scarcity of water in summer and all efforts to save water is a must.