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Purification plants implementation of heavy-metal removing bacteria

Our project aims to implement the bacterial heavy metal accumulation system in a common water purification plant. There are two phases in which such a system could be integrated : the first sedimentation step and the oxidative phase of the polluted water purification process. The integration of the system would allow the removal of nickel in a controlled step: then biomass removal would lead to an efficient purification from nickel ions. The last disinfection step guarantees the safety of this application, avoiding any release in the environment.
A typical water treatment plant consists of:

1. Screening
2. Grit removal - oil extraction
3. Primary sedimentation
4. Oxidiser tank
5. Secondary sedimentation
6. Disinfection

It is possible to integrate the bacterial system in the sedimentation tank, with the periodic removal of the superficial layers of filter sand, to remove also the bacterial component that constitutes the active biofilm to eliminate the pollutants retained by the microorganisms themselves. The flow of water must undergo three additional purification steps downstream, ensuring greater safety and further effectiveness in removing pollutants. The second possibility is to integrate the system with populations of microorganisms that compose the active sludge in the oxidiser. This has the advantage of leveraging the existing sludge elimination systems to furthermore remove bacteria, which have accumulated heavy metals from treated solution. Some operating problems may arise due to competition between the various living components of the activated sludge system. It should also be considered the environmental component like the strong stress due to variations in temperature and volume of water to be purified (that depends on rainfall) which may, as in every living system, lead to changes in the growth rate and efficiency of in vivo operation of the protein-dependent metal storage system

Sedimentation tank

One of the main elements constituting the biological treatment plant is the sedimentation tank.
The sedimentators must be dimensioned taking into account water infiltration which increases the overall magnitude , that varies considerably over the seasons. In this respect, particularly important is the flow rate influence on separation efficiency of sedimentation tanks: due to an efficiency reduction you have an excess of solids that passes from the primary sedimentation to oxidation and a loss of biological mass from the secondary sedimentation, with an increase of suspended solids and a decrease of the soluble pollutants removal efficiency that involves a worsening of the quality of the effluent solution.
It is therefore necessary to provide for install the floodways full, that allow the smooth operation of the equipment , within the limits of oversizing adopted in the design phase.
The oxidation of compounds in solution, operated by various types of microorganisms, already starts in these tanks ensured by agitation produced by mechanical stirrers (in figure: vertical axis turbines agitators).
The following passage through the oxidiser tanks involves the degradation of most of the substances present in the polluted solution. The process requires the work of the biological component of the system. Therefore, the so-called activated sludge is present in this tanks. Nevertheless, already in the sedimentation tanks, on the top of the filtration sand, it will tend to form a thick biologically active mud layer, that will have to be scraped off every now and then.

Activated sludge (oxidiser tank)

The most used activated sludge systems are called sequencing batch reactors (SRBs), term that best expresses its way of operative regime. The SRB reactors operate in cycles (usually one or more cycles per day). Each cycle consists of steps that occur in discrete periods of time. The steps are:

1. Filling
   The slurry is pumped into the reactor (aeration tank). The filling period can be static, mixed (activated sludge sediment is mixed in anoxic or anaerobic conditions with the sewage influence) or aerated.
2. Reaction
   During this period, the reactions started during the filling period are completed. The reaction period can be divided into sub-periods, that is anoxic, anaerobic and aerobic.
3. Sedimentation
   One of the main advantages of the SBRs is that the separation of the active mud takes place in the whole volume of the reactor in state of quiescence.
4. Decanting
   The treated slurry is separated from the activated sludge and unloaded from the reactor.
5. Rest
   It's the period between the filling and decanting, sometimes used in case of multi-reactors. The excess activated sludge can be eliminated in this period.

To sum up, the process expected the aeration of a mixture of screened, and primary treated sewage or industrial wastewater (wastewater) combined with organisms to develop a biological floc which reduces the organic content of the sewage. This material, which in healthy sludge is a brown floc, is largely composed of saprotrophic bacteria but also has an important protozoan flora mainly composed of amoebae, Spirotrichs, Peritrichs including Vorticellids and a range of other filter feeding species. Other important constituents include motile and sedentary Rotifers.
In all activated sludge plants, once the wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new wastewater entering the tank (return activated sludge R.A.S.). Excess sludge (surplus activated sludge S.A.S. or waste activated sludge W.A.S) is removed from the treatment process to keep the ratio of biomass to food supplied in the wastewater in balance, and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.

In the last phase, after the secondary sedimentation, the purification process finishes up with the disinfection in which the water is disinfected with chlorine in order to kill pathogens and prevent regrowth in the distribution system. From here it goes to the distribution system.

Polluted water biological treatment plant

The treatment plants are located at the end of industrial effluent or sewer networks, which may be of two types :

1. separate and distinct systems of pipes, for sewage and stormwater
2. a network unit made with a unique system in which everything flows

(you should always take into account the infiltration of ground water , which in some cases may be an important component , affecting both the quantity and quality of sewage adducts).
The flow of slurry varies during the day as a function of the rhythms of activity of the population served; these variations affect both the dimensioning of the systems, since the individual treatment units must ensure a sufficient retention time even at higher flow rates, and the operation, as appropriate precautions must be taken to always get the best treatment efficiency. Changes in the scope must also be taken into account for the sampling operations, which must depict real conditions.

The depuration plant

In the design of a water treatment plant, the hydraulic profileplays a main role, that is the succession of the levels of constituent units the plant itself, which must be calculated in such a way that:

1. the hydraulic gradient, in partial sections and overall, must to be appropriate to allow the passage of slurry through the system even at maximum flow of the sewerage or of the receiving body
2. the altitude, at which the sewage with lifting stations, must be defined

The hydraulic profile is developed based on the reception level of the receiving body, and to the topography of the site chosen for the installation of the system.
The hydraulic profile is generally calculated to achieve the flow with minimum energy consumption for lifting may be required.

We wish to thank Dr. Eng. Antonio Leanza for helping us in modeling in CatiA V5 the purification plant and for his ideas on technical applications.

References

• Roberto Passino, Manuale di conduzione degli impianti di depurazione delle acque, 1999, Zanichelli/ESAC
• D. Suresh Kumar and Dr. V. Sekaran, FBBR and MBBR Bioreactors for Sewage Treatment, International Journal of Environmental Engineering and Management ISSN 2231-1319 Volume 4, Number 1 (2013), pp. 73-88© Research India Publications

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