Page 331 - Industrial Wastewater Treatment, Recycling and Reuse

Urban Wastewater Treatment 303

process is expected to provide a better-quality effluent and require less
land area as compared to conventional sludge process because the process
is accommodated in substrate-limited conditions and also because of bet-
ter settling properties of mixed liquor. The secondary excess sludge is also
well stabilized and has better drainability for reuse.
• Conventionally, UASB technology is the suitable technology for cities of
all sizes. However, all anaerobic treatment processes including UASB
technology are very sensitive to environmental changes. Intermittent
loading can greatly affect the performance of a UASB reactor, because
the anaerobic bacteria are very sensitive to shock loading and power cuts.
The polishing pond (PP), which is the terminal unit of the scheme, is also
very crucial in deciding overall performance of the plant. The UASB
with PP technology may be a suitable option if energy recovery is the
priority. Two UASB plants of 86 and 126 MLD have been operating
at Vadodara, Gurarat, India; however, these need to be studied in detail
to assess the optimal efficiency of the treatment scheme in reducing
BOD, COD, TSS, and Fecal & Total Coliform and reuse pattern to
comply with pollution norms.
• The application of technology has constantly been upgraded over the
years. The use of aerated lagoons was common practice in the early years.
The conventional ASP gained a prominent place later and is still in use in
many places. In recent years, newer modifications of ASP have gained
importance, especially MBR for lowered land and operating cost apart
from newer versions of aerobic as well as anaerobic treatment processes.
According to the CPCB India (2005) report, most of the STPs in India
employ any one of the three technologies, namely, primary settling fol-
lowed by ASP (PST+ASP), UASB+PP, and a series of WSPs. The first
technology has been found capable of providing a final effluent having
BOD <20 mg/L and TSS <30 mg/L. The other two technologies
may provide final effluent of this quality, if operated properly. The
Gujarat State Pollution Control Board has already stipulated 20 mg/L,
100 mg/L, and 30 mg/L limits for BOD, COD, and TSS, respectively,
for treated sewage quality (CPCB, 2005). Metal removal in municipal
wastewater treatment plants can be accomplished primarily from sorp-
tion or physicochemical processes. Because metals are hydrophobic
and have a tendency to get sorbed, their removal is easier through sorp-
tion by organic material in the wastewater, even in the primary settle-
ment stage. Coprecipitation of metals also occurs with chemical
removal of phosphorus. The highest removal efficiencies are therefore