Water Quality Control

Two complementary approaches to protect water quality:

a) Legislative bodies provide the laws that regulatory agencies use to define acceptable discharges and establish standards that govern the minimum quality of water for its many beneficial uses.
b) Scientific and engineering community provides the technical guidance needed by legislators and regulars, as well as technology used to achieve those standards.

Two critical systems combine to break the carrier feces water victim sequence responsible for the spread of waterborne diseases.

a) Water collection, treatment and distribution system that provides safe drinking water. Safe Drinking Water Act (SDWA)
b) Wastewater collection and treatment system that removes contaminants before releasing them. Clean Water Act (CWA)
These 2 systems responsibility mainly is to kill pathogens in water, other than removal of toxins such as industrial solvents, HMs and pharmaceutical products. CWA also reduces BOD and nutrient loading to protect the ecology of the receiving water.

Water treatment plant → usage + industrial → sanitary sewer system + Storm sewer system (old city combine them leads to overloading) → Wastewater treatment plant (National Pollutant Discharge Elimination System, NPDES)
Purpose: change raw water into drinking water

SDWA’s basic thrust

a) Required EPA to establish national standards for drinking water quality
b) Required operators of some 16000 public water systems in the country to monitor and treat water quality if necessary.

Drinking water standards fall into 2 categories

a) Primary standards – Enforceable and based on health-related criteria [EPA, National Ambient Air Quality]
Microbiological Standards: test for fecal contamination (Coliform bacteria) than pathogenic microorganisms.
b) Secondary standards – Unenforceable and based on aesthetic and unaesthetic characteristic, color, foaming, odor, hardness, corrosivity etc.
[Maximum contaminant level (MCL), Maximum contaminant level Goal (MCLG)]

Surface Treatment plant
1) Screening & Grit removal – remove large floating, sand, grit and suspended debris.
2) Primary sedimentation – remove particles that settle out by gravity for hours
3) Rapid mixing and coagulation – use chemicals (alum) and agitation to suspend particles (colloids) into large particles by neutralize negative surface charges
4) Flocculation – formation of floc into larger particles
5) Secondary settling – remove floc
6) Filtration – removes light and small particles which gravity doesn’t helps
7) Sludge processing – dewatering and disposal of solids and liquid collected from settling tanks
8) Disinfection contact – add disinfectant (free chlorine: chlorine gas[1], sodium hypochlorite[1] or calcium hypochlorite[2]) to inactivate remaining pathogens (killing + prevent pathogen growth[add amonia]) before distributing

Alternative disinfectant: chloride, explosive + expensive and ozone, expensive but most powerful without trace
GW Treatment plant
1) Aeration – remove excess and objectionable
2) Flocculation – chemical addition that forces Ca and Mg above their solubility
3) Sedimentation – remove hard particles
4) Recarbonation – adjust pH and alkalinity but causing additional hardness causing ions
5) Filtration – removes light and small particles which gravity doesn’t helps
6) Disinfection contact – add disinfectant to inactivate remaining pathogens before distributing

GW is better in free particle and pathogens but it contain high levels of dissolved minerals or objectionable gases such as Ca and Mg.