Drinking water treatment: an overview

Drinking water treatment turns river, lake or groundwater into water that is safe and pleasant to drink, and keeps it that way all the way to the customer's tap. The aim is not sterile water but water free of pathogens in numbers that could cause illness, low in harmful chemicals, clear, and without unpleasant taste or odor. This lesson gives an overview of how that is done: the multi-barrier approach, the conventional treatment train for surface water, the treatment of groundwater, membranes and ultraviolet light, disinfection and the CT concept, disinfection by-products, distribution systems and the residuals that treatment leaves behind. The upper-year course on water and wastewater treatment designs each process in detail; here the goal is to understand what each step does and to make first sizing calculations.

Willowbend's raw water intake and reservoir job includes a new treatment plant for the city, sized for 40,000 m³/d on an average day and 60,000 m³/d on a peak day. Nadia's task is a preliminary layout: which processes, how many basins and filters, and whether the chlorine contact tank provides enough disinfection.

The multi-barrier approach

No single treatment step is perfect, so drinking water protection relies on several independent barriers, each of which reduces risk. The barriers begin at the source, with protection of watersheds and wellhead areas from sewage, manure and spills. Treatment provides physical removal of particles and microorganisms by coagulation, sedimentation and filtration, then inactivation by disinfection. A protected distribution system keeps treated water from being recontaminated, with a disinfectant residual, pressure maintained at all times and backflow prevention. Monitoring, trained and certified operators, and emergency plans tie the barriers together. Walkerton and Flint both showed what happens when several barriers fail at once.

Sources of water

Surface water from rivers, lakes and reservoirs is plentiful but carries particles, natural organic matter, algae and microorganisms, and its quality changes with seasons and storms. Prairie rivers carry heavy silt loads after snowmelt and can be very warm and algae-rich in late summer. Groundwater is usually clear, cool and of steady quality, but it may contain dissolved minerals such as iron, manganese, hardness, arsenic or uranium, and shallow wells can be contaminated by nitrate or bacteria. Groundwater under the direct influence of surface water, such as wells beside a river, must be treated like surface water because it can carry protozoa.

The conventional treatment train

Most surface water plants use the same sequence of steps, often called conventional treatment:

Natural particles, from clay to bacteria, are tiny and carry negative surface charges that make them repel one another, so they would take days or years to settle on their own. Coagulants such as aluminum sulfate (alum), ferric chloride or polyaluminum chloride supply positive charges and form sticky hydroxide precipitates that sweep particles together. The right dose depends on the raw water and is found with jar tests, in which samples are dosed and stirred in a row of beakers to see which dose gives the clearest water. Coagulation also removes much of the natural organic matter that would otherwise form disinfection by-products.

Alum reacts with the water's alkalinity as it forms aluminum hydroxide. Each milligram of commercial alum consumes about 0.5 mg of alkalinity as CaCO₃, so a soft, poorly buffered water may need lime or soda ash to keep the pH in the range where coagulation works best.

Sedimentation

In an ideal settling basin, a particle is removed if it settles to the floor before the water carrying it reaches the outlet. The analysis, due to Allen Hazen in 1904, shows that removal depends not on the basin's depth or detention time but on its surface area. Particles settling faster than the overflow rate, the flow divided by the basin surface area, are completely removed:

The overflow rate has units of velocity, often written as m³/m²·d or m/h. Real basins perform worse than ideal ones because of turbulence, density currents and short-circuiting, so designers use conservative overflow rates and provide several basins so that one can be taken out of service for cleaning.

Filtration

Rapid granular filters are beds of sand, or anthracite over sand, about one meter deep. Water flows down through the bed, and particles are caught by straining, settling and attachment within the pores, not just on the surface. As the filter collects particles, head loss rises and eventually particles begin to break through, so the filter is backwashed by pumping clean water up through the bed to fluidize it and wash out the trapped solids. The filtration rate, or hydraulic loading rate, is the flow divided by the filter area. Filtered water turbidity is monitored continuously on every filter, because turbidity spikes can signal that pathogens are getting through. Slow sand filters, which run at much lower rates and rely on a biological layer on the sand surface, still serve some small communities.

Membranes, ultraviolet light and groundwater treatment

Membrane filtration forces water through thin polymer sheets or hollow fibers with tiny pores. Microfiltration and ultrafiltration remove particles, bacteria and protozoa, and ultrafiltration also removes most viruses; nanofiltration and reverse osmosis also remove dissolved salts, hardness, nitrate and many organic chemicals, at the cost of more energy and a concentrated reject stream that must be disposed of. Ultraviolet (UV) light damages the genetic material of microorganisms so they cannot reproduce; it is especially effective against Cryptosporidium and Giardia, which resist chlorine, but leaves no residual, so it is paired with chlorine. Groundwater treatment is often simpler: iron and manganese are oxidized with air, chlorine or potassium permanganate and filtered out; hardness is removed by lime softening or ion exchange; nitrate by ion exchange or reverse osmosis; and arsenic by adsorption or coagulation. The farming town from lesson 5 would most likely use oxidation and greensand or manganese dioxide filtration.

Disinfection and the CT concept

Disinfection inactivates the pathogens that remain after physical treatment. Chlorine is by far the most common disinfectant because it is effective, cheap and leaves a residual that continues to protect water in the distribution system. When chlorine is added to water it forms hypochlorous acid (HOCl), which dissociates to hypochlorite ion (OCl⁻) with a pKa of about 7.5; hypochlorous acid is the stronger disinfectant, so chlorine works better at lower pH. Chlorine combined with ammonia forms chloramines, weaker disinfectants that persist longer and form fewer by-products, and many large systems use them for the distribution residual. Ozone and chlorine dioxide are powerful alternatives used for primary disinfection or taste and odor control.

As lesson 4 showed, disinfection depends on the product of disinfectant concentration and contact time. Regulators publish tables of the CT, in mg·min/L, needed for a given log inactivation of Giardia or viruses at each temperature and pH. Because real tanks short-circuit, the contact time used is T10, the time for 10 percent of a tracer to pass, which is estimated from the theoretical detention time with a baffling factor:

Under the EPA's Surface Water Treatment Rules, systems must achieve at least 3-log (99.9 percent) removal or inactivation of Giardia and 4-log (99.99 percent) of viruses, and the Long Term 2 Enhanced Surface Water Treatment Rule requires additional Cryptosporidium treatment for sources with higher measured oocyst levels. Well-operated conventional filtration earns credit for much of the removal, and disinfection supplies the rest.

# Preliminary sizing for a 40,000 m3/d plant (60,000 m3/d peak)
Q_avg, Q_peak = 40000.0, 60000.0          # m3/d
v_o = 40.0                                 # sedimentation overflow rate, m3/m2.d
filt_rate = 10.0                           # filtration rate, m/h
A_sed = Q_peak / v_o
print(f"Sedimentation area at peak flow: {A_sed:.0f} m2 -> 3 basins of {A_sed / 3:.0f} m2")
A_filt = Q_peak / 24 / filt_rate
n = 6
print(f"Filter area: {A_filt:.0f} m2 -> {n} filters of {A_filt / n:.0f} m2")
print(f"  with one filter backwashing, rate in others = {Q_peak / 24 / (A_filt * (n - 1) / n):.1f} m/h")
for V, BF in ((900, 0.5), (2000, 0.7)):
    T10 = BF * V / (Q_peak / 1440)        # minutes
    print(f"Contact tank {V} m3, baffling {BF}: T10 = {T10:.1f} min, CT at 1.0 mg/L = {T10:.1f} mg.min/L")

Sedimentation area at peak flow: 1500 m2 -> 3 basins of 500 m2
Filter area: 250 m2 -> 6 filters of 42 m2
  with one filter backwashing, rate in others = 12.0 m/h
Contact tank 900 m3, baffling 0.5: T10 = 10.8 min, CT at 1.0 mg/L = 10.8 mg.min/L
Contact tank 2000 m3, baffling 0.7: T10 = 33.6 min, CT at 1.0 mg/L = 33.6 mg.min/L

The program confirms the layout: three sedimentation basins of about 500 m² each, six filters of about 42 m² each, and a well-baffled contact tank of 2,000 m³. Note the check with one filter out of service for backwashing; the remaining filters must handle the full flow at a rate that is still acceptable, here 12 m/h.

Disinfection by-products

Chlorine reacts with natural organic matter to form trihalomethanes, haloacetic acids and other by-products, some of which are linked to cancer and other health effects after long exposure. The best control is to remove organic matter before chlorination by enhanced coagulation, granular activated carbon or membranes, and to move the point of chlorination after filtration. Engineers must never compromise disinfection to lower by-products, because the microbial risk of inadequate disinfection is far greater and more immediate than the chemical risk of by-products.

Distribution systems

Treated water travels to customers through pumps, reservoirs, towers and hundreds of kilometers of pipe. Distribution systems must keep pressure positive at all times, because low pressure lets contaminated groundwater enter through leaks and lets backflow occur from customers' plumbing. Water that sits too long in oversized mains or tanks loses its residual, warms up and grows bacteria and biofilm, a problem called water age. Old cast-iron mains break during deep frosts, and lead service lines release lead into the water of older homes. Utilities manage these risks with residual monitoring, flushing, corrosion control treatment, cross-connection control and long-term pipe renewal.

Residuals

Treatment produces waste streams: sludge from sedimentation basins, spent backwash water, concentrate from membranes and spent media. Water plant sludge consists mainly of the coagulant precipitate plus the solids removed from the river. It is usually thickened, dewatered and sent to landfill or land application, and backwash water is often recycled to the head of the plant after settling. Returning untreated residuals to a river is no longer acceptable in most places, so residuals handling is a significant part of plant design and cost.