The aim of this dissertation was to investigate effectiveness of using vegetable waste (VW), Raw Commercial Garden Refuse (CGRRAW) as a carbon source for denitrification of Mariannhill treated Landfill Leachate. This was achieved by testing the effectiveness of various mixes of vegetable waste and Raw Commercial Garden Refuse (75/25, 50/50 and 25/75) being used as a carbon source to remove nitrates from treated landfill leachate by doing lab experiments, then to ascertain if it was appropriate to use one of them as a substrate, based on feasibility, and how it would be applied to a larger scale, such as a leachate treatment facility.
An excess of nitrates can be harmful to the environment and can be harmful to people thus there was a need to remove these compounds from elements that would be discharged into the environment. Nitrate removal alternatives were researched. The experiment was performed according to the procedures from Standard Methods for Examination of Water and Wastewater. Tests and experiments were done to obtain results. These results were discussed to obtain conclusions to identify how effective the use of each substrate was as a carbon source to remove nitrates from treated landfill leachate.
There were five sets of batches for each of the different substrates which were being used to treat nitrates (VW, CGRRAW, 75/25, 50/50 and 25/75). The liquid treated was treated Mariannhill Landfill Leachate, from the Treated effluent tank. The experiment was carried out twice.
The samples composite were different as it depends on what was disposed of, by Dube Tradeport, at the time of sampling. This accounts for the varying results. However, it was very apparent that the patterns remain the same for each of the kinetic plots of nitrogen removal (evident in the results ‒ see figures 4-7 to 4-11). Random samples were taken as to show that, if required, any disposed vegetables that are available can be used to treat nitrate.
The nitrate level of the Mariannhill Landfill Leachate, which was the input liquid, was approximately 4000mg/l. Most readings however, began at less than 4000mg/l. This was due to the vegetable waste having a high percentage of moisture content. The leachate was inadvertently diluted. The reason for the initial nitrate readings to be low was a combination of dilution and absorbance of the nitrates by the substrate.
The vegetable waste seemed promising as a substrate seeing that the corresponding RI7
was the highest compared to all the other substrates being tested. It had roughly the same
carbon content as the rest but it did not perform as fast since the nitrate removal was inefficient. The nitrates would be fully treated but the process would take a long time to do so. The inefficiency was due to the acidic nature of the batch eluate, as it was out of the optimum range for denitrification which, consequently did not suit the denitrifying bacteria as illustrated by the low BOD5 results. Furthermore, the vegetable waste introduced more nitrates to the liquid and at the same time treats the nitrates. This was noticed in the control results as there were no nitrates initially, thereafter, the nitrate level began to rise. On average the resulting vegetable waste batch eluate did not comply with any of the tested parameters of the DWAF standards of discharge (excluding VWA that did not have any ammonia). If the final nitrate levels were not taken into consideration, the vegetable waste substrate arguably made the leachate worse than it was initially, with regards to the DWAF discharge standard parameters. Hence, vegetable waste in this form was not a good substrate to be applied on the full scale (i.e. Applying all vegetables as disposed of, by Dube Tradeport, as a substrate to treat nitrates in a Fixed Bed Reactor) as different samples do have different results.
There have been previous experiments to determine if CGRRAW would make a suitable nitrogen removal substrate. These have yielded results similar to the findings of the experiments conducted for this dissertation. The denitrification results were acceptable as there was full treatment and done within similar time frames to previous works. It would be expected that leachate would take one to two weeks to treat nitrates of high concentrations in landfill leachate. The rates of denitrification were inconsistent. The CGRRAW substrate consists of a good portion of carbon content. However, there was not as much readily available carbon determined from the low RI7 value. There was carbon but it was not readily available for the medium amount of denitrifying bacteria to use (from BOD5, Figure 4-13).
The pH conditions were near to being within the optimum range. The CGRRAW batch effluents were not within the DWAF discharge standards, with the exception of the pH levels. The parameters were however closer to what was required than most of the substrates.
The mixes all had improved BOD5 and IR7 results which seemed to strike a balance between the vegetable waste lack of denitrifying bacteria and the CGRRAW lack of readily available carbon. The carbon content and carbon to nitrogen ratio was maintained. The pH value was also balanced such that all the batch eluate pH values for the mixes were within the optimum range for denitrification. This resulted in the treatment of nitrates to become more efficient, with the exception of the 50/50A mix. However, all the characteristic parameters tested were not in compliance with DWAF discharge standards.
The 75/25 mix was the only substrate to show a pattern of a third order polynomial (all the other substrates demonstrated a second order polynomial pattern). The denitrification rate was improved but not the most effective seen in other results. The RI7 reading would seem to be the reason for the rate not being as fast.
The 50/50 mix was very unpredictable as it did not perform as its characterisation suggested.
It was unexpected that the results would not be a balance between the 75/25 and 25/75 mixes. (50/50A was slower than all the mixes and 50/50B was as fast as 25/75)
The 25/75 mix was the fasted to treat the nitrates completely and was done so consistently.
The rate of denitrification was also the best. The 25/75 batch eluate was not with the DWAF discharge standards but was closer to complying compared with the other substrate results, similar to CGRRAW.
In conclusion, the 25/75 mix was the best substrate that can be used on a larger scale to create a horizontal constructed wetland but would require further treatment to be discharged.
This was due to 25/75 mix achieving a better rate of treatment of nitrates and has similar results with regards to DWAF discharge standards (pH, COD, NH3 and NOx). A random mix of vegetable waste, on its own, was not suitable to treat nitrates. Even though it was able to fully treat nitrates, it did not improve the leachate in terms of the other parameters of the DWAF discharge standards. The COD levels were of particular concern as they were consistently very high for all substrates. All objectives have been met and the aims have been achieved.