Biological Denitrification using Saw Dust as the Energy Source

Published on Nov 30, 2023

Abstract

Nitrate contamination is one of the major problems in groundwater, which is increasingly becoming a threat to groundwater supplies. Nitrogen in groundwater also results from human excreta, ground garbage and industrial effluents, particularly from food processing plants. In order to remove nitrate from a water source, such as ground water the biological denitrification technology can be applied for which a carbon and energy source is needed. The aim of this study was to construct an easy to operate reactor to treat the NO3 contaminated (ground) water to such qualities that it can be used as drinking water for cattle.

To achieve this objective it was investigated whether saw dust as an alternative carbon source for the bacterial denitrification, could be used. For this purpose a low maintenance reactor, which was inoculated with anaerobic digester sludge, was fed with nitrate rich (100-200 mgN/L) water. A 100% nitrate removal was observed when the feed water contained 200 mg/• NO3-N at a Hydraulic Retention Time (HRT) of 1 day, operating a stable reactor system. It was shown that similar nitrate removal rates were obtained when the reactor was operated at room or mesophilic temperatures

Denitrification is a process in which the oxidised nitrogen substances, i.e. nitrates and nitrites are reduced to nitrogen gas, such as N2O and N2, when a proton donor (energy source) is available. In most biological denitrification systems, the nitrate polluted waste water (e.g.domestic sewage) contains sufficient carbon (organic matter) to provide the energy source for the conversion of nitrate to nitrogen gas by the denitrifying bacteria. To treat groundwater, in which the nitrate contents may be as high as 100 mg NO3-N/ℓ with low dissolved carbon content, an additional proton acceptor is required. Generally, methanol is used as well as a more complex product such as molasses. In order to promote nitrate removal in remote areas, it is not advisable to use complex mechanical, maintenance requiring equipment .

Robertson as well as Robertson and Cherry and Blowes demnstrated passive in situ nitrate removal methods that are mechanically simple and do not require significant additional maintenance. They showed the use of nitrate reactive permeable sub surface barriers to passively remove nitrate from septic system effluents. These barriers were installed as layers downstream of conventional septic systems infiltration beds and as vertical walls intercepting horizontally flowing septic system plumes. The barriers contained waste cellulose solids, such as sawdust and leaf compost, which provided the carbon source for heterotrophic denitrification . It is envisaged that in South Africa a market exists for an easy to operate nitrate removal system, which can effectively reduce the nitrate concentration of the ground water to levels acceptable for the drinking water of cattle and other farm animals.

Feed Water

Artificial nitrate rich water was used as feed water for the continuous operated reactor, which contained initially 100 mg/ℓ nitrate (NO3-N), thereafter 200 mg/ℓ. During the first period of the study (till day 109) a weak NaHCO3 solution was added to the feed water in order to maintain the reactor pH between 7 and 8. Once the denitrification process was in operation, the addition of the NaHCO3 solution was omitted. Reactor(s) During the first part of the experiment, one reactor was operated consisting of a Perspex reactor (R1) of which the volume was 2ℓ.

The feed water entered the reactor at the bottom, while the effluent was discharged at the top of the reactor. The HRT was kept at 24 h during the first 109 days of the experiment. This reactor was operated at 30 °C. After 110 days of the experiment, a second reactor (R2) was introduced. This reactor was operated under the same conditions as the first reactor, however the second reactor was operated at room temperature. Reactor R1 was inoculated with micro organisms, obtained from the anaerobic digester at the Daspoort Sewage Plant, Pretoria, South Africa. Reactor R2 was inoculated with half of the biomass from Reactor R1.