The water treatment plant should produce continuous supply of safe drinking water regardless of raw water

Quality variation, the main part of the water purification is the filtration, so all pretreatment processes are to maximize the efficiency of filtration

Unit Design parameter

Parameter setting

controlledvariable

disturbance variable

Manipulatedvariable

Rapid mixing Velocity gradient 530sˉ¹ Alum ,polymerChlorine dosages

Influent feed rate

Dosing flow rate

Mixing time (40 sec-60sec)

flocculation Velocity gradient 100Sˉ¹ fixed fixed

Detention time 40minutes

clarifier Surface Over flow speed,(Surface loading rate)

1.2m/hr Fixed

fixedWeir overflow rate 10m²/hr Fixed

Retention time >2.5 hourfixed

Rapid sand filter Filtration rate 11m/hr Mid post chlorine

Influent flow rate Dosing flow rate

Head lossAnd turbidity

Automatic Effluent flow rate

Disinfection Residual free chlorine (0.05-2 )mg/l Residual free chlorine analyzer

Chlorine dosage flow rate

In rapid mixer after adding the Alum coagulant, mass balance on suspended solids leads to effluent particle concentration, in flocculator section the size of particles will increase depends on the G parameter (velocity gradient)

In clarifier the ratio between the settling velocity of the particles to the surface over flow parameter will determine the efficiency of the clarifier ,but the settling velocity depends on the sizes of particles generated during the flocculation and this due the coagulants and chlorine addition in flash mixer ,so the pretreatment activity before the

Filtration, is optimized by quantity of chemical additions since the other parameters in the table above are fixed as design values ,the turbidity measurement is high time delay process spatially when let the effluent filtered water turbidity to be in closed loop with coagulants flow rate addition, so the flow rate corrections will be manual as in the drawing (

Clarifier size and its diameter/height ratio is designed for one pump (30-P3A/B/C) to achieve low surface over flow rate

The filtration is fully automatic process; variation of filtration rates can achieve high efficiency and low water turbidity

The filtered water rate is controlled indirectly(constant flow rate control-constant head) , Influent flow is splitted bymeans of an influent weir box above the maximum water level of filter @754.2m altitude and minimum water level @752.07m altitude, so each filter receives an equal (or nearly equal) portion of total flow

a level sensor in each filter sends a signal to the effluent control valve which opens or closes to maintain constant head, When the filter is first placed in operation after backwashing valve to close, As the filter head loss builds up the water level above the filter media will tend to rise, causing the valve to open, until the effluent control valve is fully opened this is the time for filter back washing

At a minimum, both head loss and effluent turbidity should be monitored on each individual filter. Head loss is monitoredby measuring the differential pressure between the effluent line(upstream of the control valve) and the top of the filter media. The pressure signal will initiate an alarm on high level, and in automatic, initiate a filter backwash cycle. On high-high level, the filter should shut down until the operator has investigated theCause of the high-high alarm and/or manually initiated a backwash. Turbidity should be monitored by an online turbidity meter, for each filter. As with head loss, a high turbidity set point can initiate an alarm (and possible backwash cycle), and a high-high turbidity set point should shutdown the filter to prevent poor quality water reaching the clear well. Back wash loop:Control of the flow rate may be manual or automatic. Manual control may be used on straight backwash systems (. Manual control requires adjustment to a throttling valve on the backwash supply until the desired flowrate, as measured by the flow meter, is achieved. This valve should be locked in this position and adjusted seasonally to compensate for water temperature, Excessive and undesirable disturbance of the media can be caused by a sudden rush of water on pump start-up or rapid valve opening. This is avoided by having bypass valve Backwash flow rate can be varied by using variable speed drive on the backwash pump, using an in-line flow control valve, a flow transmitter on the backwash supply will provide a signal to the local PLC. The PLC will use this signal to control the pump speed/valve position.This control is closed loop automatic control, the initiating of back wash can be manual or automatic depends on many parameters such as effluent turbidity, high water level above the filterMedia , max head loss, running time, number of filters in backwash ,

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