Page 435 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

390 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

management of products, services, and production or SCADA: An acronym for ‘‘supervisory control and data
delivery processes to ensure the attainment of oper- acquisition.’’ Refers to data acquisition by elec-
ational performance, product, or both in keeping tronic sensors for such quantities as pressure, tem-
with quality requirements (Symons et al., 2000, perature, specific electric conductivity, pH,
p. 384). turbidity, particle counts, dissolved oxygen, specific
Quality control (QC): A system of functions carried out at a ions, flow meters, etc. Signals are 4–20 mA and are
technical level for the purpose of maintaining and proportional to the sensor reading and are transmit-
documenting quality. It includes such features as ted to an analog board. The analog board transmits
personnel training, standard operating procedures, the signal to a computer where it is interpreted in
and instrument calibrations (Symons et al., 2000, terms of a calibration to read the actual value, for
p. 384). example, 7.5 mg=L dissolved oxygen. Control is by
Rapid filtration: Rapid filtration is defined here as a technol- actuated valves that generally have on-off functions
ogy that provides the means to implement the ‘‘depth but may, alternatively, regulate water flow. The
filtration’’ process. The technology includes the computer sends a signal to a micro-dac board
under-drain system, the backwash system, the waste- which contains a micro-relay. For an open valve
water system, the instruments, and the control sys- the signal requires the relay to permit current to be
tem to permit the depth filtration process to work; transmitted to the valve to perform the function
coagulation is the most important part of the process. intended.
Rate-of-flow controller: A valve in the effluent pipe from a Single collector: A term that arises often in discussions of
filter under-drain system that regulates flow at a modeling is the ‘‘single collector.’’ The term ‘‘single
given set-point by opening or closing the valve, collector’’ refers to a single constricted cell, Happel’s
usually in small increments. The valve has associ- sphere-in-cell, etc., that by their nature exclude the
ated with it a flow meter and that provides the signal presence of neighboring grains. The effect is that the
for valve adjustment. At the start of the filter run, viscous flow field, for example, as well as other
with the water level in the filter box at full depth, the fields of a complex system, vis-à-vis a single col-
valve is near-closed and takes up most of the head- lector, are not included in the model.
loss of the system. As the filter clogs, the valve opens Slow sand filtration: Slow sand filtration is characterized by
further, reducing the valve headloss. As the run low hydraulic loading rate, with consequent large
approaches termination, the valve opens fully, at filter area and the raw water without coagulation is
which point all of the headloss occurs in the media applied to the filter. Headloss increase is due to a
and none in the valve. For a declining rate filter surface deposit on the top of the filter, called the
scheme the rate-of-flow controller valve is not schmutzdecke. At terminal headloss, the bed is dewa-
needed, nor is it needed for a constant flow, rising tered to just below the bed surface and the schmutz-
water level scheme. In every case a flow meter decke is removed by scraping. Slow sand filtration is
should be installed in the pipe from the under-drain characterized by both straining and depth filtration.
system for each individual filter. Straining occurs as ambient particles deposit on the
Regime: A prevailing pattern of behavior or the pattern of a surface of the filter bed, forming a schmutzdecke.
regulated system. Adapted from American Heritage Depth filtration occurs as a portion of the ambient
Dictionary. particles penetrate the filter bed. If the filter has been
Remnant water: Remnant water is defined as backwash in operation for some months, the granular particles
water that remains in the filter box after backwash will have developed biofilms which then serve as
and includes water in the under-drain system, pore collectors. At the risk of being simplistic, we might
water, and headwater. say that in rapid filtration we condition the particles
Ripening: The first stage of a filtration cycle in which the (by coagulation) and in slow sand filtration we con-
effluent turbidity or particle concentration declines dition the collectors (by hoping for or promoting
with time toward a ‘‘steady state’’ level, say 0.05 growth of biofilms). Slow sand is a distinct technol-
NTU or say 10 particles=mL. ogy and is the topic of Chapter 13.
Saturated zone: At the influent end of the filter (the top in Specific solids deposit: The mass of solids deposit per unit
most cases), the influent flux of floc particles is volume of filter bed, that is, pore volume plus solid
3
received. As these particles adhere to the media volume; units are kg solids per m filter volume.
grains and previously deposited particles, they Standard temperature pressure: The most common stand-
occupy void space causing a higher interstitial vel- ards for temperature and pressure are 258C and 101
ocity with a proportionately increasing rate of 325 kPa (1.00 atm), respectively; this condition is
shear. As the rate of shear increases, the rate of designated STP, meaning standard temperature and
detachment of particles at some point equals the pressure. In U.S. Customary units, the values are
rate of attachment. At this point the filter bed is 688F and 1.00 atm. In Europe, the common term
‘‘saturated.’’ used is, ‘‘normal’’ temperature and pressure (NTP).