Environmental Engineering Reference
In-Depth Information
100%
50%
80%
40%
36%
32%
60%
30%
40%
20%
16%
16%
20%
10%
0%
0%
<0.03
0.03-0.1
0.1-0.4
0.4-0.5
Manganese (mg/L)
FIGURE 22.8
Distribution of manganese levels in surface water sites in southern regions of Ghana.
rain shower, runoffs bring increased amount of silt to the surface water bodies resulting
in increased turbidity and levels of both iron and manganese. Water treatment systems
installed at these locations need to be capable of handling the seasonal variation in iron
and manganese levels, and support personnel, including operators, need to be trained
to proactively identify the problem and adjust the process parameters to make sure that
water dispensed from the center does not exceed the allowable limits for these contami-
nants. The job of monitoring and controlling the process would have been made easier
if there was a low-cost technology available that was either not sensitive to wide luctua-
tions in levels of iron and manganese or incorporated a closed loop feedback mechanism
using low-cost inline analyzers.
In the interior regions of India, groundwater provides a sustaining source of raw water.
These are typically contaminated with dissolved salts, primarily bicarbonates of calcium
and magnesium along with cations such as sodium and potassium and anions such as
chlorides, sulfates, and nitrates. In the WaterHealth sites in India, elevated levels of luo-
rides are seen in some regions of Andhra Pradesh and Karnataka. In the latter state, ele-
vated levels of nitrates, iron, and manganese and marginally elevated levels of arsenic are
also common.
Generally, reverse osmosis is adapted as a universal means of treating groundwater for
higher levels of dissolved salts. In WaterHealth's experience, this technique also provides
a cost-effective method for reducing luorides and marginally higher levels of arsenic
instead of other adsorbent-based methods such as use of activated alumina. Compared
with the use of adsorbent material that needs to be regenerated using corrosive chemi-
cals, reverse osmosis is easier to operate and does not require frequent monitoring to look
for luoride breakthrough. Reverse osmosis is an energy-intensive process and involves
discharge of signiicant amounts of water as a waste stream. While designing the system
for achieving the right balance between cost of equipment and energy use, overall water
recovery may need to be kept in the 75%-80% range, which still results in 20%-25% of the
water discharged as a waste stream.
For situations where water does not have high levels of dissolved salts but has elevated
levels of arsenic or luoride, a commonly used option is to use an adsorbent such as acti-
vated alumina for arsenic or luoride and ferric oxide-based adsorbent for arsenic. The
problem with these adsorbents is that once they get depleted, they have to be sent to land-
ills for disposal. Even though manufacturers make all efforts to make sure that the spent
adsorbents pass the regulatory leaching requirement, conditions at landills may be such
