Environmental Engineering Reference
In-Depth Information
dissolved organics that also addresses taste, odor, and color arising from these contami-
nants. Passage of water through ine 5- and 1-micron ilters improves the clarity and drops
the turbidity of water to <0.1 NTU.
Steps (i), (ii), and (iv) may also be replaced by treatment through an ultrailtration (UF)
membrane where reduction in turbidity to <0.1 NTU can be achieved with a single iltra-
tion step. Generally, UF membranes are tolerant to high levels of inluent turbidity (200-
300 NTU) and need to be maintained carefully to prolong their life. The trade-off with
replacing steps (i), (ii), and (iv) with UF is generally the cost. For surface water use, a good
UF design should include a controller for auto backwash as the fouling of the membrane is
rapid and frequent back low is required to dislodge the particles. The combined cost of a
good-quality UF module and the controller exceeds the cost of equipment needed for steps
(i), (ii), and (iv). The one-step iltration achieved by UF considerably reduces the equipment
footprint and enables the system to be operated without the intervention of an operator
while delivering consistent quality water downstream of the UF.
Treatment steps described thus far primarily affect the physical characteristics of the
water and do not alter the water chemistry. Depending on whether higher than desired
levels of dissolved solids are present in the water, a treatment step using reverse osmosis
membranes is included where water is further puriied to reduce the levels of dissolved
salts. This step is energy intensive and is accompanied by discharge of wastewater that
may contain high levels of salt. WaterHealth systems are carefully designed to balance
equipment cost with energy cost and wastewater discharge.
As a inal treatment step, water is disinfected using UV radiation before it is sent to over-
head storage tanks. Disinfection using UV is preferred because it is cost-effective and does
not involve handling or addition of chemicals such as sodium hypochlorite that are cor-
rosive in nature and require special handling and storage. Addition of chemicals has the
risk of secondary reaction with constituents in the water to produce products such as halo-
methanes or haloacetic acids when halogens are used as disinfectants. These by-products
are carcinogenic and require that addition of halogens be tightly monitored and controlled.
Disinfection by UV does not have this problem, as there is no health issue related with UV
overdose of water. A disadvantage of UV disinfection is that it does not provide residual
disinfection. Generally, at WaterHealth sites, any form of residual disinfection is unneces-
sary as the water collected by consumers is used up within a day or two. As a practice,
WaterHealth offers an option to the users to purchase a 20-liter can designed to prevent
secondary contamination and allow easy dispensation. The ill opening in the WHI con-
tainer is narrow so that no vessel can be introduced and the container features a dispensing
valve at the bottom. At places where likelihood of secondary contamination is high owing
to the nature of open vessels brought by consumers, WaterHealth introduces trace amounts
of chlorine to provide residual disinfection. At these sites, since the water is already disin-
fected, the amount of chlorine needed is extremely small. WaterHealth utilizes safer cal-
cium hypochlorite where available and supervises the chlorine addition very closely.
In the countries where WaterHealth has operations, availability of continuous power is
the biggest challenge. One way WaterHealth has overcome this challenge is to design over-
head tanks where treated water is stored so that water can be dispensed to the consumers
with the aid of gravity even when power is not available.
22.2.4.1 UV Waterworks™: WaterHealth's UV Disinfection Technology
WaterHealth is harnessing the strengths of UV Waterworks' design in providing safe
water to communities. UV Waterworks consists of a part-cylindrical water low channel
