Treatment of Domestic Sewage Using Algal Bioreactor

...

• Minimizing energy usage.

• Minimize land area usage.

• Ensure the greatest penetration of light into the fluid to maximize photosynthesis.

Following characteristics would be possessed by the species of algae that would be ideal for this process:

• Removes Phosphorus and Nitrogen effectively

• Not highly influenced by the presence of bacteria

• Has a comparatively higher growth rate

• Is able to grow on sewage easily

• Can be conveniently harvested

3.2 Key variables of the process:

Algae rely on a number of essential parameters for growth. Light, temperature, P, N, C, trace nutrients and pH. Any of these could be a rate limiting factor for growth. The balance of these parameters is affected by many factors. Where nutrients are in excess then CO2 or light could easily be the limiting factor. Any number of scenarios can exist that place each variable as the limiting factor. Maintaining the correct balance for a specific species is the most important aspect of creating optimal conditions.

- Light:

A minimum intensity of light is required as a source of radiant energy for photosynthesis to take place. There is an ideal light intensity and wavelength required by algae to grow which vary from specie to specie, but as a general rule, the more the better. We have to come up with a system which uses minimum energy and maximum penetration of light. The penetration also depends on the concentration of algae or the turbidity of the water.

Former research at IIT Delhi has tried to implement some of the following and this project will also be based on the findings of the same:

- Use of Photosynthetically Active Radiation (PAR) instead of normal light: PAR essentially denotes the wavelengths of light from 400 nm to 700 nm. The idea behind using this is that it is known that the photosynthetic organisms only use these wavelengths for carrying out photosynthesis. So, if only the required wavelengths are used, then a lot of cost of energy can be reduced.

- As time passes, the turbidity of the liquid will increase due to increased algal concentration causing lower penetration of light. This can be overcome by either using a thin bioreactor or using a bioreactor having a maximum exposed surface area (like the horizontal prism-shaped bioreactor). Apart from these, other methods can be tried such as using enclosed strips fixed with LED lights which can be inserted inside the bioreactor. This will ensure that the whole bioreactor is illuminated and light does not become the limiting factor of growth. While experiments have been carried out earlier using this system which have given good results, still the problem of a large number of LED lights used persists which may cause heating of the system and larger energy requirements. So, a new system of illumination which includes the use of optical fibers is proposed. An optical fiber is an extremely thin fiber made up of silica which can be used to transmit light between two ends of the fiber. Preliminary experiments have shown that much less number of LED lights will be needed for the same amount of illumination acquired by using just the LED strips. Also, since the fibers are ultra-thin, they will take up much less volume inside the bioreactor. For the sake of comparison, experiments using both, LED lights and optical fibers would be carried out.

[pic 7][pic 8][pic 9]

[pic 10]

[pic 11]

[pic 12]

[pic 13][pic 14]

- Temperature:

Algal cells also have an optimal working temperature. Temperature has little effect when light is limiting. When light is not limiting, increase in temperature can increase the rate of photosynthesis, growth/doubling rates are consequently increased

- Nutrients:

The major nutrients of concern to algal growth are consequently those which are important for all plants; Carbon, Nitrogen, Phosphorus. Sulfur and Silicon can also be added to the list. Minor nutrients tend to be metals and are only required in trace amounts; most are toxic at larger concentrations. However, the iron content of the medium may have a significant effect on the algae’s growth.

Generally in these all are present in appropriate amount in wastewater and can be removed by using algal bioreactors.

However it is generally noted that CO2 can be deficient in wastewaters in comparison to their P and N content. So additional CO2 supplementation may be needed to promote algal growth inside the reactor, since algae require CO2 to grow. But this will only be done if there is no substantial reduction in the C, N and P content. This can not only help in the growth of algae but sparging of CO2 could also help in proper mixing inside the reactor which in turn will help in each algal cell receiving proper light to grow even if it was earlier in a low light penetrated area.

- Bacteria:

Algal growth is promoted in systems in which there is bacterial residence. This is because of the production of CO2 by the bacteria which is taken up by the algae as a source of carbon. They are also responsible for the heterotrophic biodegradation of mixed organic load.

- pH:

pH is an important factor for algal growth, CO2 supplementation can be introduced to improve growth rates and control pH at an optimal value.

- ALGAL GROWTH AND HARVESTING:

The project started during the summer break during which the task of algal growth was taken up. A sample of algae collected from the Hauz Khas Lake nearby was taken and was employed for growth. The sample contained a combination of species of algae. The Applied Microbiology Lab at the Centre for Rural Development and Technology (CRDT), IIT Delhi was occupied to carry out the growth process. At the beginning, 1L of algae was grown which was sub-cultured to 2L. This was further sub-cultured till a total of 8L was obtained with suitable