Biomedical Engineering Reference
In-Depth Information
picture for researchers and engineers to better understand the mechanism of uncoupled energy
metabolism as well as to develop efficient strategies for sludge reduction in practice.
2. Strategies for Minimizing Biomass Production
Nowadays, various technical means have been developed for minimizing biomass
production, such as sludge alkaline thermal treatment [8], ozonation [9,10], chlorination [11],
advanced oxidation process [12], enzyme-induced biological hydrolysis [13], biodegradation
of sludge in membrane-assisted reactors [14,15], predation on bacteria [16,17], oxic-settling-
anaerobic (OSA) process [18,19], chemical uncoupler-associated activated sludge process
[20,21], etc. All these strategies could be roughly classified into four major categories as
specified in the following sections.
2.1. Lysis-cryptic Growth
This strategy is to use physical, chemical or biological forces to induce cell lysis and
release cell contents (e.g. organelles, proteins, DNA, RNA, lipids, etc.) into the medium as an
internal food source for wastewater microorganisms, thus leading to the increase of system
organic loading (Figure 1). This amount of organic loading is usually defined as
autochthonous substrate, and will be reutilized in microbial metabolism. Part of
autochthonous substrate will be assimilated into biomass to build up cell matrix, while part of
them will be released as respiratory products, resulting in a reduction of biomass production.
The biomass growth by absorbing the autochthonous substrate and by taking the original
organic substrate always happen simultaneously and cannot be distinguished one from the
other, and is therefore termed as cryptic growth [22]. There are two linked processes in lysis-
cryptic growth, including lysis and biodegradation. Between these two steps, lysis of
particulates to soluble substrate is the rate-limiting stage, because the cells usually have a
protein-lipid bi-layer plasma membrane, forming a barrier separating cell contents from the
surroundings, which is resistant to lysis. Therefore, an increased cell lysis efficiency can
contribute to the overall decrease of sludge yield [5,23].
It is known that moderate osmosis through simply lowering the ionic strength of the
medium is usually sufficient enough to lyze most cells which induces them to swell, burst and
finally release their contents into the solution [24]. So far, several physical, chemical or
biological methods have come into practice to enhance the complete dissociation of the cells
and their cellular components to reduce sludge burden, including (i) physical disintegration
using sonication, mechanical mills, mechanical agitation with glass or ceramic beads,
freeze/thaw cycles and nitrogen bomb/nitrogen burst cell disruption method (Table 1); (ii)
chemical disintegration applying alkaline thermal treatment, ozonation, chlorination or
advanced oxidation processes with Photo-Fenton reagent and (iii) biological method utilizing
enzyme-induced biological hydrolysis, etc [5].
Ozone is a very strong chemical oxidant with a remarkably short life span that has
potential to reduce excess sludge in CAS process. During ozonation, part of recycled sludge is
contacted with ozone to be mineralized to carbon dioxide, water and biodegradable organics
which will be further decomposed in the subsequent biological treatment unit (Figure 2) [1].
