Membrane and Desalination Technologies - TCE Moodle Website

378 N.K. Shammas and L.K. Wang
Disposal options for MF/UF backwash residuals are similar to those for conventional water
treatment plants, and typically include the following:
1. Discharge to a suitable surface water body
2. Discharge to the sanitary sewer
3. Treatment with supernatant recycle and solids disposal
The discharge of backwash residuals to surface water bodies or the sanitary sewer is likely
to be subject to state and/or local regulations and, in the case of surface water discharge, to
require a permit. Moreover, the potential to utilize one of these options may be complicated if
the residuals include chemical wastes. In addition to the use of coagulants added to the feed,
some backwash procedures utilize chlorine or other chemicals. Small amounts of chlorine
may be quenched and acids or bases can be neutralized prior to discharge, although larger
amounts or other types of chemicals added to the backwashing processes may require
additional treatment or preclude discharge to a surface water body or sanitary sewer.
On-site treatment options for MF/UF backwash residuals are also similar to those that
might be used with conventional media filtration, and include clarification, sedimentation
lagoons, gravity thickening, centrifuging, belt filter presses, or a combination of these
processes (26). A second stage of MF or UF may also be utilized to further concentrate
residuals and increase process recovery. If a sedimentation process is used to treat MF/UF
backwash residuals, the addition of a coagulant may be necessary to improve the settling
characteristics of the solids if coagulant is not already applied in the MF/UF pretreatment
process. With on-site treatment, the supernatant is generally recycled to the treatment plant
influent while the concentrated solids are transported off site for landfilling or other means
of disposal. As with discharging, the addition of chlorine or other chemicals to the
backwash process may complicate residuals treatment. It is important to note that backwash
residuals concentrate any pathogenic organisms that are present in the feed water, as well as
other suspended solids. Although the control of such concentrated pathogens in filter
backwash residuals is largely unregulated, the potential treatment of this stream should be
taken into consideration if these residuals are to be discharged into a surface water receiving
body.
6.2. Chemical Cleaning Residuals
Both MF/UF and NF/RO membranes undergo periodic chemical cleaning, and thus both
types of systems generate spent chemical waste as a byproduct of these processes. As with the
backwashing process for MF/UF, the frequency of chemical cleaning varies on both a siteand
system-specific basis. Although chemical cleaning is conducted much less frequently
than backwashing, the frequency is also more difficult to predict. Generally, MF/UF systems
are cleaned no more frequently than once every month for efficient operation and the
minimization of system downtime, although it is not uncommon for these systems to operate
for much longer without requiring chemical cleaning. The cleaning frequency for NF/RO
may vary anywhere from 3 months to 1 year, depending on the feed water quality and the
effectiveness of feed water pretreatment for minimizing fouling. However, because chemical