4. Results & Discussion
4.3 Nutrient Optimization
FTRW is a chemically created industrial wastewater with virtually no naturally occurring nutrients.
For this reason the nutrients required for optimal biomass growth in the AnMBR had to be identified and added - and since nutrients are a significant contributor to operating cost – their individual concentrations had to be optimized. Initially only N and P were dosed (days 1 to 90, Figure 4.1), this was later supplemented by a nutrient mix optimized for biological phosphate removal (Wentzel et al., 1988). However it was not until the incorporation of macro and micro nutrients as suggested by Du Preez (1987) for the anaerobic digestion of acetic acid, that a significant increase in system performance was observed (day 120, Figure 4.1). This nutrient mix proved sufficient; however a significant concentration of N and P was measured in the effluent, which gave rise to questions on the further optimization of the Du Preez nutrient mix. This commenced the second phase of the nutrient optimization study, i.e. finding optimal concentrations of the various species in the nutrient mix.
These optimal concentrations were identified by varying the individual nutrient concentrations over extended periods of time (>30 days) to observe the effect on the system OLR. As discussed in Chapter 3, the OLR is a direct response of the reactor performance; if the effluent SCFA was high (>
150 mgAc/L; poor/deteriorating reactor performance) the OLR is decreased and if the effluent SCFA is low (< 150 mgAc/L; increasing/high reactor performance) the OLR is increased. The extraction of the effects of the individual nutrients on the OLR proved complicated. After much consideration the following approach was adopted: Since there are a large number of parameters that can have an effect on the OLR a high/low OLR at a high nutrient concentration does not give any valuable information. However, if the OLR remains low for an extended period of time - if a nutrient concentration is low - then the probability of this nutrient having an effect on the OLR is high. The effect of the low nutrient concentration was then confirmed by increasing it and observing its effect on OLR recovery. The individual nutrient concentrations vs. OLR are plotted for the entire 685 day dataset, as presented in Figure 4.2 for Sulfate (as S).
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0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
Influent Sulphate [mgS/L]
Organic Loading Rate [kgCOD/m3 /d]
Figure 4.2, Influent Sulfate (S) vs. Organic Loading Rate (OLR)
For sulfate (as S), high OLR and a low (<4 mgS/L) influent S concentration were never simultaneously observed. However when a threshold value of 4 mgSO42-
-S/L was reached, the reactor performance increased rapidly.
Nutrients are required for biological growth, which means that the nutrients consumed forms part of the cell mass and MLSS of the reactor. However it was found that the actual concentrations of macro nutrients like Phosphates (P), Sulfide (S) and Iron (Fe) are so low in the MLSS that the nutrient requirements are virtually independent of sludge age. This further validates the individual-nutrient- concentration vs. OLR technique for nutrient optimization.
This optimization study was done for all the macro nutrients and also for yeast extract and the micro nutrient Nickel. The OLR-nutrient-concentration scatter plot for each of these species can be seen in Appendix 4.2. The results of this study are presented in Table 4.1. In contrast to other nutrients, anaerobic biomass does contain a significant amount (~10 %) of Nitrogen (N) and sludge age has a significant effect on the AD-FTRW system’s N requirements. The relationship between influent nitrogen (Nti) and sludge age can be expressed as:
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. . .
r t n
ti s b b
s i
V X f
N N N N
= + = R Q + (4.1)
Where
Nti = Influent Nitrogen Concentration [mgN/Linfluent] Ns = Influent N concentration that has become bound
in the biomass harvested from the system per day [mgN/Linfluent] Nb = Minimum background liquid N concentration
for uninhibited growth [mgN/Linfluent]
Vr = Reactor Volume [L]
Xt = MLSS Concentration [mgTSS/L]
Rs = Sludge Age [d]
Qi = Influent Flow Rate [L/d]
fn = Nitrogen Fraction in Biomass [mgN/mgTSS]
Nb is the minimum “back ground” concentration for uninhibited N uptake by the anaerobic biomass.
Unfortunately, nitrogen was always dosed in excess to the AnMBR, so the Nb concentration could not be quantified. However, from earlier research done on the pilot-plant AnPBR it would appear that a Nb = 25 mgN/L is required (Table 4.1). Correspondingly, for a 23 L reactor volume, 25 gTSS/L solids concentration, 300 day sludge age and an influent flow of 23 L, the influent nitrogen (Nti) should be ~50 mgN/L.
Table 4.1, Optimized Nutrient Concentrations for the AD-FTRW
Influent Nutrient
Abbre- viation
Du Preez Nutrients' Concentration
[mg/L]
Optimized Concentration [mg/L]
Nitrogen Nti 252 [Vr.Xt.fn]/[Rs.Qi] + Nb
Phosphate Pti 57 10.0
Sulfate SO4-Sti 23 4.0
Calcium Cati 3 <1
Magnesium Mgti 13 <1
Yeast Extract 54 <1
Iron Feti 11 1.0
Nickel Niti 0.002 <0.001
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If the ‘Du Preez nutrients’ is compared to the nutrient requirements optimized for the ADMBR is compared, it is can be noted that the optimized nutrients are significantly lower in all of the cases. A possible reason for this is very long sludge ages (>100 days) and resultant low sludge wastage on the ADMBR, this will be discussed in detail in Chapters 5 and 6. The nutrients N, P, S and Fe are of primary importance for the AD-FTRW and should be dosed as macro nutrients. In contrast, the study has shown that Ca, Mg, and Yeast extract are not required as macro nutrients. Even at µg/L levels, Nickel (Ni) does not seem to have any effect on the reactor performance.