1. Biological Denitrification using Saw Dust as the Energy Source

2. Let’s start by defining Denitrification…

3. Denitrification is the process of removing nitrate(NO3) or nitrites(NO2) from ground and/or surface waters by reducing them, in presence of a electron donor, to nitrogen gas(N2).
The electron donor, usually carbon, is referred to as the energy source.

4. But isn’t nitrate important for life?
Nitrate is a form of nitrogen usually found in the soil. Most crops need nitrogen as it forms part of DNA and proteins in plant and animal cells. Hence in the right amounts, nitrogen is essential to all forms of life.
Crops usually obtain it by absorbing it in form of nitrates from the soil while animals get it from feeding on plants.

5. Major Sources include:
Nitrogen-fixation from atmosphere by lightning, leguminous plants & N-fixing bacteria.
Fertilizers
Manure
Municipal wastewater

6. Why then denitrify?
As a result of leaching, nitrates usually find their way to groundwater.
When found in concentrated amounts in ground water and surface water, such water can have harmful effects both to humans and animals if used for drinking.

7. Drinking water high in nitrates concentration results to an interference with one’s red blood cells ability to transport oxygen in the body.
The result to both humans and animals is an illness known as methemoglobinemia, mainly found in infants.
Hence there is need to monitor groundwater used for drinking and carry out de-nitrification if found to have excess nitrates.

8. The maximum limit, as set by the World Health Organization is:
For humans: 10 mg/ℓ NO3
For animals: 100 mg/ℓNO3

9. Apart from biological de-nitrification, are there other methods that can be used?

10. Other processes can be used such as:
1. Physical processes – Reverse osmosis, ion exchange and electrodialysis
2. Chemical de-nitrification
PROS’
Highly Efficient
CONS’
High operating costs
PROS’
Highly Efficient
CONS’
Ammonia is formed

11. Advantages of Biological Denitrification:
High nitrate removal efficiency,
Moderate operating cost
High process stability and reliability
The process can be controlled with ease

12. Biological Denitrification Process
The raw water to be denitrified is fed into a bioreactor (with the presence of denitrifying bacteria-innoculum)
Most wastewater to be denitrified contains enough carbon(energy) source for the process and thus with the right conditions, nitrates are converted to N2 gas through a number of reduction steps:

13. Nitrates are first converted into nitrites, then to nitric oxide, then to nitrous oxide and finally to nitrogen.
NO3- →NO2- → NO → N2O → N2
Nitric oxide, nitrous oxide and nitrogen are gaseous products and they can be safely released into the atmosphere.

14. Commonly used denitrifying bacteria include:
Lactobacillus
Pseudomonas
Achromobacter
Brevibacterium
Aerobacter
Bacillus
Proteus
Flavobacterium
Alcaligenes
Micrococcus

15. If water to be treated is deficient in dissolved carbon, then additional carbon(energy) source/ electron donor must be supplied.

16. Commonly used energy sources include:
Methanol
Ethanol
Acetate
Glucose
Molasses
Lactate

18. Sawdust as an Energy Source
Research has shown that biodegradable solid waste can be used effectively as a carbon(energy) source.
Research carried out on the effectiveness of sawdust for raw water containing mgN/L, which is a probable value for groundwater, has shown nitrates removal of up to 100%
The usability of sawdust as a carbon source presents a great advantage since it is readily available as a waste product of the forest industry.

19. Sawdust is mainly composed of cellulose, hemicellulose and lignin.
Of the three, Cellulose forms the greatest percentage and is the major source of carbon.
For sawdust to be used as an energy source, a special set of bacteria capable of degrading cellulose, needs to be availed.

20. Examples of bacteria that can degrade cellulose:
i) Fermentative anaerobes, e.g. Clostridium, Ruminococcus & Butyrivibrio,spp.
ii) Aerobic bacteria, e.g. Cytophaga, Sporocytophaga, spp. Cellulomonas & Thermobifida, spp.

21. These bacteria degrade cellulose to form products such as methane, acetate, ethanol which avail the carbon necessary for de-nitrification.

22. What are the optimum conditions for this bio-reaction?

23. pH
Different microorganisms require different pH levels to thrive. The optimum pH for de-nitrification has been found to be between 6-8.

24. Temperature:
Temperatures of between 20-30˚C have been found to be optimum.

25. Hydraulic Retention Time:
=24 Hour

26. Mode of Operation:
Batch or Continuous reactor models can be used, with anaerobic conditions.
For a continuous process, the feed rate should not be very high.

27. When the right conditions are observed, raw water with a nitrates concentration of approx. 200mg/ℓNO3, can be de-nitrified with 100% efficiency within 50days.

28. Keep Safe