Clean Water Technology, Inc.

Synergistic application of advanced primary and secondary wastewater treatment systems

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Courtesy of Clean Water Technology, Inc.

Published in Water and Waste Digest membrane issue, November 2008
Miroslav Colic; Chief Scientist, Clean Water Technology Inc., Los Angeles, CA To be published in Water and Waste Digest later this year
The increased regulatory pressures and demand for reused water stimulated development of several new wastewater technologies in the last decade. Advanced treatment systems are characterized by small footprint, high contaminant removal efficiency, flexibility, resilience to changes in wastewater quality and high level of automation.

To recycle/reuse wastewater, most contaminants have to be removed. Primary systems are used to remove suspended and dispersed solids, free and emulsified oils, fats and grease. Secondary systems (biological treatment) are used to reduce organic contaminants (BOD’s, TOC). Tertiary systems such as filtration and membrane filtration are used to remove what is left after secondary treatment (ions, small non - biodegradable molecules, very fine solids etc.)

Several hybrid systems, combining two or three different technologies have recently been developed and applied. For instance, membrane bioreactors (MBR) are composed of biological unit (aeration and microbial growth) responsible for the biodegradation of the waste compounds and a membrane filtration module for the separation of produced biosolids. Membranes can be immersed in the aeration unit, or used externally. Cleaning of the membranes is achieved through frequent permeate back pulsing and occasional chemical washing. Membrane bioreactors have been pilot tested and used in industrial and municipal wastewater treatment. Another configuration under development is using fixed film bioreactors such as moving bed biofilm reactors (MBBR) followed by external membrane ultrafiltration.

Moving bed biofilm reactors (MBBR) are a hybrid of activated sludge and biofilter processes. Contrary to most fixed film bioreactors, MBBR utilizes the whole tank volume for biomass. However, contrary to activated sludge reactors, MBBR does not need return activated sludge (RAS). This is achieved by having a biomass grow on plastic high surface area carriers that move freely in the water volume of the reactor kept within the reactor volume by a sieve arrangement at the reactor outlet. At the bottom of the tank, large bubble aeration system assures mixing and floating of plastic carriers with attached biomass.

The biofilm carrier is made of high density polyethylene (0.95 g/cm3) and shaped as small cylinders with a cross on the inside of the cylinder and “fins” on the outside. The original cylinders have a length of 7 mm and diameter of 10 mm. Later, various shapes and sizes were introduced by numerous manufacturers. One of the important advantages of the moving bed biofilm reactor is that the filling fraction of carrier in the reactor may be subject to needs. That means that by increasing the filling fraction one can increase surface area and capacity of the reactor to reduce BOD’s without additional tanks. Microorganisms growing on such media are also much more resistant to pH and toxic shock as well as fluctuations in BOD’s. Produced biosolids are easy to separate and dewater.

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