Top 5 Types of Waste Water Bio-treatment

The following points highlight the top five types of waste water bio-treatment. The types are: 1. Aerobic Bio-Treatment 2. Anaerobic Bio-Treatment 3. Advanced Bio-Treatment 4. Molecular Techniques in Waste Water Treatment 5. Metals Removal by Microorganisms from Waste Waters.

Waste Water Bio-Treatment: Type # 1.

Aerobic Bio-Treatment:

Aerobic processes are often used for munici­pal and industrial waste water treatment.

Easily biodegradable organic matter can be treated by this system.

The basic reaction in aerobic treatment plant is represented by the reactions (1, 2):

Microbial cells undergo progressive auto- oxidation of the cell mass:

Cells + O₂ → CO₂ + H₂O + NH₃ (2)

Lagoons and low rate biological filters have only limited industrial applications. The pro­cesses can be exploited as suspended (activate sludge) or attached growth (fixed film) sys­tems.

Aeration tanks used for the activated sludge process allows suspended growth of bacterial biomass to occur during biological (secondary) waste water treatment, while trickling filters support attached growth of bio- mass.

Advanced types of activated sludge sys­tems use pure oxygen instead of air and can operate at higher biomass concentration. Biofilm reactors are applied for waste water treatment in variants such as: trickle filters, rotating disk reactors, air-lift reactors.

Domes­tic waste waters are usually treated by aero­bic activated sludge process, since they are composed mainly of proteins (40-60%), carbohydrates (25-50%), fats and oils (10%), urea, a large number of trace refractory organics (pes­ticides, surfactants, phenols).

Waste Water Bio-Treatment: Type # 2.

Anaerobic Bio-Treatment:

Anaerobic treatment of wastewater does not generally lead to low pollution standards, and it is often considered a pre-treatment process, devoted to minimization of oxygen demand and excessive formation of sludge.

Highly concen­trated wastewaters should be treated anaerobically due to the possibility to recover energy as biogas and low quantity of sludge.

Research and practices have demonstrated that high loads of waste water treated by anaerobic tech­nologies generates low quantities of biological excess sludge with a high treatment efficiency, low capital costs, no oxygen requirements, methane production, low nutrient require­ments.

Waste Water Bio-Treatment: Type # 3.

Advanced Bio-Treatment:

Advanced waste water bio-treatment must be considered in accordance with various benefi­cial reuse purposes as well as the aspect of human and environmental health. This is es­pecially important when the treated wastewa­ter is aimed to use for the rehabilitation of urban creak and creation of water environment along it.

Membrane technology is considered one of the innovative and advanced technolo­gies which rationally and effectively satisfy the above mentioned needs in water and waste water treatment and reuse, since it combines biological with physical processes.

In combi­nation with biological treatment, it is reason­ably applied to organic waste waters, a large part of which is biodegradable. In fact, this is the combination of a membrane process like microfiltration or ultrafiltration with a sus­pended growth bioreactor.

It is widely and successfully applied in an ever increasing number of locations around the world for municipal and industrial waste wa­ter treatment with plant sizes up to 80,000 population equivalent (Membrane Separation Activated Sludge Process, MSAS). The process efficiency is dependent on several factors, such as membrane characteristics, sludge charac­teristics, operating conditions.

A new genera­tion of MSAS is the submerged type where membrane modules are directly immersed in an aeration tank. This aims to significantly reduce the energy consumption by eliminat­ing a big circulation pump typically installed in a conventional MSAS.

Membrane bioreactors (MBR) can be applied for removal of dissolved organic substances with low mo­lecular weights, which cannot be eliminated by membrane separation alone, can be taken up, broken down and gasified by microorgan­isms or converted into polymers as constitu­ents of bacterial cells, thereby raising the qual­ity of treated water.

Also, polymeric substances retained by the membranes can be broken down if they are still biodegradable, which means that there will be no endless accumu­lation of the substances within the treatment process. This, however, requires the balance between the production and degradation rates, because the accumulation of intermediate metabolites may decrease the microbial activi­ties in the reactor.

MBRs can be operated aerobically or anaerobically for organic compounds and nu­trients removal. Due to its hybrid nature, MBRs offer advantages and gain merits.

The main advantages of biological processes in comparison with chemical oxidation are:

a. No need to separate colloids and dispersed solid particles before treatment.

b. Lower energy consumption, the use of open reactors, resulting in lower costs.

c. No need for waste gas treatment.

Waste Water Bio-Treatment: Type # 4.

Molecular Techniques in Waste Water Treatment:

Although molecular technique applications in waste water bio-treatment are quite new, be­ing developed during the 1990s and not ap­pearing to be more economical than the estab­lished technologies, major applications may include the enhancement of xenobiotics re­moval in wastewater treatment plants and the use of nucleic acid probes to detect pathogens and parasites.

Among these techniques, clon­ing and creation of gene library, denaturant gradient cell electrophoresis (DGGE), fluores­cent in situ hybridization with DNA probes (FISH) were proved to be most interesting. Waste water treatment, processes can be im­proved by selection of novel microorganisms in order to perform a certain action.

However, the use of DNA technology in pollution control showed to have some disadvantages and limi­tations such as:

a. Multistep pathways in xenobiotics biodeg­radation

b. Limited degradation, instability of the re­combinant strains of interest in the en­vironment, public concern about deliber­ate or accidental release of genetic modi­fied microorganisms, etc.

Waste Water Bio-Treatment: Type # 5.

Metals Removal by Microorganisms from Waste Waters:

Heavy metals come in waste water treatment plants from industrial discharges, storm wa­ter, etc. Toxic metals may damage the biologi­cal treatment process, being usually inhibitory to both aerobic and anaerobic processes.

How­ever, there are microorganisms with metabolic activity resulting in solubilisation, precipitation chelation, bio methylation, volatilization of heavy metals. Metals from waste water such as iron, copper, cadmium, nickel, uranium can be mostly complexed by extracellular polymers produced by several types of bacteria (B. licheniformis, Zooglearamigera).

Subse­quently, metals can be accumulated and then released from biomass by acidic treatment. Non-living immobilized bacteria, fungi, algae are able to remove heavy metals from waste water.

The mechanisms involved in metal re­moval from waste water include:

a. Adsorption to cell surface

b. Complexation and solubilisation of metals

c. Precipitation

d. Volatilization

e. Intracellular accumulation of metals

f. Redox transformation of metals

g. Use of recombinant bacteria.

For example, Cd²⁺ can be accumulated by bacteria, such as E. coli, B. cereus, fungi (As­pergillusniger). The hexavalent chromium (Cr⁶⁺) can be reduced to trivalent chromium (Cr³⁺) by the Enterobacter cloacae strain; sub­sequently Cr³⁺ precipitates as a metal hydroxide.

Some microorganisms can also transform Hg²⁺ and several of its organic com­pounds (methyl mercury, ethyl mercuric phos­phate) to the volatile form HgO, which is in fact a detoxification mechanism. The metabolic activity of some bacteria (Aero monas, Flavobacterium) can be exploited to transform selenium to volatile alkylselenides as a result of methylation.