Analysis of Monitoring Data for the year 2000
Discharges of permitted substances are monitored and reported to the EPA's Permit Compliance System. Nitrogen discharges are reported as minimum, maximum, and/or average reports. Because the maximum monthly discharge is the most consistently reported measurement, it is used in the following analysis. Reports for some facilities are missing or incomplete. Where possible, missing numbers are calculated from, or estimated based on remaining reports from that facility. Where some measurements were missing from columns, the average of the remaining reports was used. Where total nitrogen and total ammonia (as N) was reported, the difference was taken to be the nitrite (as N) plus nitrate (as N) value. Where nitrite (as N) and nitrate (as N) were not reported separately, the nitrite (as N) plus nitrate (as N) value was assumed to be 25% nitrite (as N) and 75% nitrate (as N) based on those facilities that did report separately. The resulting values are not precise, but sufficient for the purposes of this thesis.
The following chart shows the total nitrogen concentrations (year 2000 average of reported monthly maximums) discharged by the WWTFs. In some cases, these concentrations are reported as total nitrogen. In other cases they are reported separately as ammonia (as N), nitrite (as N), and nitrate (as N). Facilities may also report one number for ammonia (as N) and another for nitrite plus nitrate (as N).
However, concentrations are only part of the story. The following chart shows the year 2000 average of reported monthly averages of daily flow.
With reports for average flow in million gallons per day and concentrations in mg/liter, we can find the percentage of total nitrogen loading in lbs/day, as shown in the following chart.
This chart shows that the combined nitrogen loadings from NBC Field's Point and NBC Bucklin may dominate the system. While the loading from Upper Blackstone to its receiving waters is comparable to that of NBC Bucklin and Fields Point, it's impact on the Providence and Seekonk Rivers will be diminished by attenuation factors.
But again, total nitrogen loadings are only part of the story. The importance of nitrification is worth repeating here:
... it is especially important for facilities to provide nitrification. Not only is ammonia toxic to fish, but if ammonia is allowed to enter the receiving water, it will be nitrified by nitrifying bacteria there. This nitrification process is extremely oxygen intensive (using 4.2 mg of oxygen for each 1 mg of ammonia converted) (WPCF, 26) and can result in an oxygen sag downstream of the facility. When this process is complete, which can happen very quickly under the right (worst) conditions, the ammonia will have been transformed to nitrate, which fertilizes algae, which die and decompose, which further reduces dissolved oxygen. (WPCF, 16) Therefore, the oxygen demand of discharged ammonia and nitrite must be counted twice - once as it is nitrified - and then again when it produces biomass which must then be decomposed.
Since the components of total nitrogen are often permitted, and thus reported, separately, it is possible to compare the concentration of ammonia in the discharge from several of the WWTFs to the concentration of nitrite plus nitrate. The results are shown in the following graph:
It seems evident that some facilities are not performing adequate nitrification. The magnitude of these discharges can be seen when the average flow is factored in, giving the results in lbs/day loadings as shown in the graph below:
It is not possible to judge from these figures the precise implications of the ammonia releases. Are the conditions right in the Blackstone, Seekonk and Providence Rivers for the bacterial nitrification of ammonia to take place quickly? Is this amount of ammonia loading enough to cause the levels of oxygen depletion that have been detected in the Providence and Seekonk Rivers? Do the physical factors of these estuaries allow for susceptibility of the combined effects of nitrification and decomposition? These questions will need to be answered by water quality models, but it does seem clear that as a first step, nitrification processes should be considered for some of these facilities. If all ammonia and nitrite released were to fully nitrify to nitrate in the receiving waters, and if all nitrate produced by this nitrification and released through direct discharge were to be converted to biomass and then decomposed by bacteria, the total oxygen demand of the discharge from each WWTF would be shown by the following chart and graph. The results of the same analysis with 2001 discharge data is also shown.
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See Oxygen demand calculations
The numbers shown here are useful only for purposes of illustration. The actual oxygen demand put on receiving waters, and more specifically, on the Providence and Seekonk Rivers, by wastewater treatment facility discharges would depend on a number of factors including temperature, salinity, amount of ammonia used directly as fertilizer by algae, grazing of algae by marine animals, uptake of nitrogen by other aquatic plants, amount of nitrogen trapped by sediment, and other attenuation factors (natural physical, chemical and biological processes.)
The changes in maximum oxygen demand for each facility from 2000 to 2001 can be seen in the following chart.
Although some of the facilities have implemented changes to reduce their nitrogen discharges, it is difficult to say exactly what the changes shown by the discharge data tell us. The maximum oxygen demand calculations for each year were performed using the available months of data. While the data for 2001 is fairly complete, the data for 2000 is not. Many of the facilities were missing some reports and some had only six months of data or less, so we don't know how the missing data would affect these changes. In addition, facilities are not consistent in the form of nitrogen discharge that is reported. The following chart shows the difference in concentration for each form of nitrogen reported.
It is important to consider concentrations here because permits limits are expressed in concentrations and changes in operations and technology will be measured by changes in concentrations. However, attempting an analysis of this chart presents two problems. First, nitrogen discharges are monitored and reported in different combinations of the five forms shown. As the chart shows, some facilities report only one and some report up to four of the five forms. In addition, I have used only the maximum monthly reports because it is the value reported most consistently, but adding maximum monthly reports in their various forms may not accurately reflect the actual total nitrogen discharge. Second, this chart shows only the changes for which there are discharge reports for the same month in both years. While in some cases, this allowed the use of 12 data points, in others it allowed as few as three. Because the character of discharges can vary significantly from month to month, the inconsistency in the number of data points can distort the results. Complete and consistent monitoring is necessary for an accurate analysis of discharges from wastewater treatment facilities.