The less time it takes to remove all the nitrates, the more effective that particular mix, of vegetable waste and immature compost, is as a carbon source for nitrate removal. It has been found that combining vegetable and garden waste can significantly improve the effectiveness of nitrite removal.
Introduction
Motivation
Aims
Objectives
Dissertation Layout
Literature Review
- Health effects of Nitrates and Nitrites
- Environmental effects of Nitrates
- Acidification
- Cultural eutrophication
- Direct toxicity of inorganic nitrogenous compounds
- Mariannhill landfill Conservancy
- Living Earth
- Dube TradePort
- Important composite in Leachate
- Heavy Metals
- pH
- Ammonia
- Nitrites
- COD
- RI 7 test (Respiratory Index after 7 days)
- Leachate Management
- Nitrate drinking water standards
- Discharge standards
- Nitrogen cycle
- Nitrification
- Ammonification
- Nitrogen Fixation
- Denitrification
- Current Method of removing Nitrates from Leachate
- Fixed-bed reactors
It is important to measure the pH because it has a major effect on the denitrification rate. Nitrate in drinking water is measured in terms of the amount of nitrogen present or in terms of both nitrogen and oxygen.
Methodology
Sampling
- Leachate
- Vegetable waste
- Commercial Garden Refuse (CGR RAW )
- Cone and Quartering
- Different mixes between vegetable waste and CGR RAW
Immediately, the fresh vegetable waste was mostly butternut, celery, jam squash, sweet potato, carrot and onion. This was done with clippers and knives for the vegetable waste and a garden grinder was used for CGRRAW.
Moisture Content, Total Solids and Volatile Solids Tests
RI 7 test (Respiratory Index after 7 days)
Liquid tests: Setting up the substrate as a liquid (eluate)
Total Carbon, Total Nitrogen and Carbon/Nitrogen ratio
Nitrates and Nitrites
Ammonia Test and NO x test (Using the distiller)
After the indicator solution changed back to its original color, the volume of HCl was recorded by the digital titrator.
Chemical Oxygen Demand (COD) test
BOD 5 test
The BOD5 test was conducted in BOD bottles that were dark and closed with Aqualytic caps. The experimental output is the BOD5 concentration in mg/l and is displayed directly on the screen of the sensomat. The more biodegradable substances in the sample, the more carbon dioxide is released, the greater the pressure change and the higher the BOD (mg/l).
Batches tests
The solid sample was sent to Bemlab for total carbon and total nitrogen testing. This experiment was performed twice for two different substrate samples, which were mixed to form the five substrate mixtures.
Results and Discussion
Introduction
Characterisation of Leachate
Characterisation of Solid Substrates
Most of the nitrogen content has similar values; however, there was a disparity between VWA and VWB, this was similar when considering carbon/nitrogen results. The samples were similar, in that a large percentage of vegetables were present in similar proportions in both samples, but some vegetables were present in only one of the samples. RI7 and C/N values for VWB both appear significantly greater than all other substrates.
The bar chart below has been plotted to more easily illustrate the nature of the RI7 measurements. Past experience shows that CGRRAW takes a few days to decompose, after which the carbon begins to become more readily available. This means that the results were not comparable to be absolutely certain of the accuracy of the result (such as p>0.05, which is usually used to determine if the results were statistically acceptable, but due to the lack of uniformity in the results , which was due to the unstable nature of the substrate, these results were accepted).
A limited number of replicate tests could be run at a time, and because the rate of degradation was rapid, the time to complete the tests was limited. The plant debris was unique to those tested previously, as the amount of readily available carbon allows the bacteria present to work faster.
Characterisation of Eluate
- pH
- COD
- BOD 5
- Ammonia
- NO X
As you can see from the graph above, the more vegetable waste present, the lower the pH level, increasing the acidity in the mixture. Therefore, most eluates are unlikely to be in the best pH range for denitrification. The prominent percentage in the 75/25 mix was vegetable waste, which subsequently resulted in lower acidity (indicated in Figure 4-2).
The substrates in Experiment A have a pattern that shows (with respect to the mixtures) that the more CGRRAW present in the mixture, the higher the BOD level. Experiment A illustrates that a certain plant waste sample does not cause an increase in ammonia levels in the eluate. The mixtures show a pattern in which the ammonia content decreases as the amount of plant waste in the mixture decreases.
Bioleaching should not occur in the VWB batch test as it does not convert the ammonia to nitrates (as shown in Figure 4-5). The more vegetable debris there was in the mix, the higher the NOX test readings.
Batch Experiment results
- Introduction
- Vegetable waste
- CGR RAW
Acclimatization in this sense refers to the system inside the batch bottle trying to find a balance for the rate of denitrification, as the ammonia has converted to nitrates; nitrates are converted to nitrites which eventually convert to nitrogen gas. Biologging contributed to the fluctuations (as seen in Appendix B), where the nitrate levels in the control increased from 0-1000mg/l. However, there was no ammonia from the eluate characterization tests, but considering the ammonia results from the Mariannhill leachate leads to the inference that VWA treated the ammonia (by converting it to nitrates) and then co-treated it.
This is reflected in the results as the time equal to the zero reading averaged 850 mg/l against an actual leachate reading of 4000 mg/l. Instead, according to the characterization of the eluate and the results of the VWB batch, it caused the ammonia concentration to rise to very high mg/l levels). These results, while showing improvements in most parameters, still assert that further processing may be required to make the output acceptable to DWAF discharge standards.
The trends of these plots differed from the others in that the nitrate level initially did not change much (within a day or two), but then the denitrification rate increased for about three days and then began to asymptotically approach zero. Therefore, the 75/25B substrate scavenged nitrates as fast as they were released by bioleaching, and this was done rapidly by the presence of nitrite.
Total time to Denitrify
Batch Characterisation Results
- Batch pH Results
- Batch BOD 5 Results
- Batch COD Results
- Batch Ammonia Results
- Batch NO X Results
Each of the mixtures had a higher level of BOD than its constituents, vegetable waste and CGRRAW combined. An overall pattern emerged when comparing the substrates with the same constituents of experiments A and B (eg VWA to VWB). The substrate with the lower COD measurement is always achieved Tend in a short time.
With the exception of VWA, all the batch experiments resulted in a higher concentration of ammonia than the leachate started with. The results do not meet the DWAF standards, with the VWA being the only exception. However, the batch results still do not meet the DWAF discharge standard of 15mg/l NOx.
The substrates with the lowest NOx concentrations were CGRAW and 25/75, which contained the least vegetable waste. As can be seen from the graph, a pattern emerged: the more vegetable waste in the substrate, the higher the NOx concentrations.
Application to a larger scale
The substrate could be supplied from the local garden waste disposal contractors and the vegetable waste is already available at Dube Tradeport. There is a tremendous amount of garden waste available locally as the Durban area has a sub-tropical climate and Dube Tradeport has daily waste from the packaging facility. The horizontal constructed wetland will be a large concrete tub filled with substrate and covered with aggregate to prevent the substrate from drifting away and for the process to be kept anaerobic.
It would be more feasible to use a 25/75 mixture with plant waste, as this substrate has the most efficient denitrification process with similar adverse effects as the next best substrate. Any selected substrate would still require polishing if the liquid resulting from the treatment is intended for discharge into a freshwater body.
Conclusions
The inefficiency was due to the acidic nature of the batch eluate as it was outside the optimal range for denitrification, which in turn did not suit the denitrifying bacteria, as shown by the low BOD5 results. On average, the resulting plant waste batch eluate did not comply with any of the tested parameters of the DWAF effluent standards (except VWA, which did not contain ammonia). These produced results similar to the findings of the experiments conducted for this dissertation.
The effluents from the CGRRAW batch were not within the DWAF discharge standards, with the exception of the pH values. This has made the treatment of nitrates more efficient, with the exception of the 50/50A mix. The batch eluate 25/75 did not meet DWAF discharge standards, but was closer to compliance compared to the other substrate results, similar to CGRRAW.
This was due to the 25/75 mixture achieving a better nitrate treatment rate and having similar results regarding DWAF discharge standards (pH, COD, NH3 and NOx). Although it was able to completely treat nitrates, it did not improve the leachate in terms of other parameters of the DWAF discharge standards.
Recommendations
A polishing treatment should be investigated for the effluent that will result from the runoff if one of the substrates tested in this dissertation is applied, particularly for COD removal, as all substrates tested showed high COD levels. It could also be temperature versus nitrate removal, but it was less likely since the lab was kept at a relatively constant temperature. Therefore, further research should be conducted to investigate the effect of these parameters on nitrate removal (rates).
Bibliography
Nitrate removal using unripened compost and pine bark as a carbon source: column simulation, Durban: University of Kwa-Zulu Natal. DENITRIFICATION OF LAW USING MUNICIPAL WASTE AT DIFFERENT LEVELS OF STABILITY: SIMULATIONS IN A GROUP TEST, Durban: University of KwaZulu-Natal. DENITRIFICATION OF HIGH STRENGTH NITRIFIED LANDFILLS USING COMMERCIAL PRIVATE LAW AND LIGHTLY COMPOSTED GARDEN REFUSE (CGR) AS A CARBON SOURCE, Durban: UKZN.
Appendix
Appendix A (initial Characterisation)
Any recommendations in this report are based on the assumption that the samples were representative of the bulk from which they were taken. Any recommendations in this report are based on the assumption that the samples were representative of the bulk from which they were taken.
Appendix B (nitrate readings)
Appendix C (Batch eluate characterisation)
Appendix D (equations used in Appendix)
Appendix E (Summary of all Results)
