The odour control system installed at Capella Drive consists of a biological filter followed by an activated carbon filter to enable “polishing” of any residual odour remaining after the biological stage. SA water has asked Bioaction (the manufacturer of the system) to review the performance of this filter in an effort to determine the operating efficiency of the biological stage and report on any effects that the current performance would have on the lifetime of the secondary “polishing” activated carbon system.
SA water has provided data detailing, over a period of 21 days, the levels of H2S present in various stages of the filtration system. The data gathered took place over a summer period between 15/8/2017 and 5/09/2017. All the supplied charts have been placed in Appendix A. The key data points from these charts has been summarised in the below table:
Activated Carbon Inlet
This data shows results that are typical of a dual stage system with the first stage biofilter removing a large percentage of the H2S and as a result reducing the average ppm(v) from 5.35 down to 0. However again in a typical result we can see here that the biofilter has allowed for a peak of 3ppm(v) to pass through. While the activated carbon filter removes any of the remaining H2S.
Biofilters require a population of biomass in order to treat an incoming contaminate, in this case H2S, in order to be effective. If the biomass has not built up to a level required for an incoming contaminate to be fully removed, then additional biomass is grown. Similarly, if there is not enough contaminate to sustain the biomass it will die back. Due to the time dependent nature of biomass growth biofilters in their nature are slow reacting and as a result generally do not handle “peaks” in the incoming contaminates as effectively as chemical and adsorption based filters.
It can also be seen in this data that the activated carbon vessel was effectively able to remove any additional contaminates remaining after the biofiltration stage.
Based on the provided data it is possible to calculate the percentage reduction of H2S across the biofiltration stage. This data was then used to produce a plot showing the removal efficiency of the biofilter over time. However due to the rapid changing of data the results are noisy and as such a noise filter utilising the moving average technique was implemented. A moving average filter smooths data by replacing each data point with the average of the neighbouring data points defined within a span. In this case a span of 20 has been used for all instances of filtration. The introduction of this filter allows for a more visual determination of the trends that occur within the data. A copy of these graphs has been included in Appendix B.
The calculation of the removal efficiency uses the following equation:
Where C_i is the concentration in while C_0 is the concentration at the outlet. In this case, we can see that the lowest removal efficiency occurs at the highest peak with an efficiency of 50%. In addition, there is an overall average of 99.93% removal efficiency during the period of testing.
One of the issues most evident throughout this data is the resolution of the instruments used is limited to ±1 ppm(v) which when reading the low values shown in this data there is a large number of 0 ppm(v) readings that could potentially skew the averages.
Based on the supplied data there is an average of 0.007 ppm(v) H2S at the inlet to the activated carbon filter. This number is low enough that based on mass flow calculations of H2S entering the system at a flow rate of 280L/s an expectation can be made for a carbon lifetime of >10 years. This calculation however only accounts for the presence of H2S and there are other compounds and contaminates that will certainly affect and reduce this lifetime. It is also not likely that the carbon would be able to last this long due to the “Shelf life” of the activated carbon and its impregnates.
Based on the provided data a conclusion can be made that the biofilter is operating at a very high efficiency achieving an average removal efficiency of 99.93% with a lowest efficiency of 50% occurring during the highest peak during the sample period. This breakthrough is typical of biofilters due to their slow reacting nature and as such it can be said that this biofilter if performing sufficiently.
A conclusion can also be made that the activated carbon filter is able to remove all the remaining H2S contaminates with a resulting reading of 0 ppm(v) at the vent stack for the entire period of testing. In addition, the carbon lifetime should not have any appreciable reduction as a result of “peaks” in the incoming air stream as the average H2S loading is only 0.007 ppm(v).