The effluent chemical oxygen demand (COD) measurements were normalised to the control pots to ensure that the response seen was only for the shock loading of COD from the toxins dosed. As all the pots were fed from the same feed reservoir it was assumed that the normalised response seen was due only to the shock loading of the toxin and any other effects were also seen in the control pots which would be accounted for in the normalisation. The shock dose of COD varied for each of the toxins (Table 6-6).
Table 6-6 – COD shock dose from each toxin
Toxin COD (mg)
Bleach 167
Shower gel 1835
Washing powder 566
Phenol 4739
Sodium Dodecyl Sulphate 452
Zinc sulphate 170
Two distinct responses to the spot doses were observed for effluent COD.
Bleach, SDS and zinc sulphate had no discernable effect on the effluent COD with no change greater than 20 mg.l-1 compared to the control pots (Figure 6-8 and Figure 6-9). However, washing powder, shower gel and phenol showed a rapid increase to a peak at 1 HRT (6 hours) followed by a gradual decline over the 4 HRTs (24 hours) monitored, without returning to control pot levels.
102 -100
0 100 200 300 400 500 600 700 800
-5 0 5 10 15 20 25
Effluent COD (mg.l-1)
Time (Hours)
Shower Gel Washing Powder bleach SDS phenol zinc sulphate
Figure 6-8 –Effluent COD vs time for all six toxins tested normalised to their respective controls
In the case of the phenol, a shock load of 4739 mg of COD (Table 6-6) was applied at time t = 0 (Figure 6-8). The total normalised effluent COD over the 24 hours monitored was 7530 mg COD meaning that 2791 mg COD was released as a result of the action of the phenol on the biomass. The origin of the COD could be a mixture of soluble and insoluble COD. This is clearly different from the effluent concentration expected from an ideal reactor with a non reactive tracer (Section 6.3.3.1) and it can be assumed that the phenol interacts with the biomass. The phenol dose has had an extreme effect on the COD removal of the system and would result in system failure.
2 HRT 4 HRT
1 HRT
103 -40
-20 0 20 40 60 80 100 120
-5 0 5 10 15 20 25
Effluent COD (mg.l-1)
Time (Hours)
Shower Gel Washing Powder bleach SDS zinc sulphate
Figure 6-9 – Effluent COD excluding phenol normalised to their respective controls
The washing powder dose resulted in a shock load of 566 mg COD with a resulting 759 mg COD in the effluent, a positive difference of 193 mg COD. The effluent turbidity showed an increase indicating degreasing and separating dirt through surfactants is the most likely action here, with an increase in colloidal particles in the effluent. The evidence does not suggest any toxic action as the other parameters monitored do not show that the biomass was harmed in any way.
Similarly the shower gel produced a shock load of 1832 mg COD, however, the effluent COD over the 24 hours monitored was 880 mg with a removal rate of 52%. Although there is evidence that the biomass has been disturbed there is clearly a part of the community that is still functioning and removing substrate.
This indicates true inhibition or kill of a part of the biomass, which is unexpected as the dose for shower gel was 2.5% of the EC50 value (1 ml.l-1 dosed compared with an EC50 by respirometry of 38.8 ml.l-1).
1 HRT 2 HRT 4 HRT
104 6.3.3.3 Effluent Ammonia
None of the products contributed to the ammonia loading. Neither the sodium dodecyl sulphate nor the zinc sulphate had any adverse effects on the effluent ammonia with normalised readings for SDS being a maximum of 0.52 mg.l-1 difference and zinc sulphate being a maximum of 1.33 mg.l-1 different from the control.
Most notable amongst the effluent ammonia results are those of the shower gel and phenol. Both were constant for the first hour of sampling with levels of 1.3mg.l-1 above the control for shower gel and a gradual increase for phenol to just 0.34mg.l-1 above the control. At the six hour mark, however, both had increased to over 6 mg.l-1 above their respective controls. This gap widened at the 12 hour sample to phenol being at 11.24 mg.l-1 above and shower gel to being 8.6 mg.l-1. The effluent ammonia for the phenol dosing peaked at this point whereas the shower gel increased again at 24 hours to 11.05 mg.l-1 above the control. This gave an absolute effluent ammonia of 13.96 mg.l-1. Influent ammonia levels were around 30 mg.l-1 for the shower gel giving a minimum removal rate of 53 %. Influent levels were 16.4 mg.l-1 for phenol, resulting in the lowest removal rate being just 31%.
It is well known that nitrifiers are more sensitive than other parts of the microbial community (Pagga et al., 2006) and these two dosing experiments demonstrate inhibition of ammonia removal and both show that it takes longer than 4 HRTs for the system to return to pre dosing levels. The lag in reaction time for the shower gel indicates that parent compounds present were degrading into more toxic byproducts (Petersson et al., 2000).
105 -8
-6 -4 -2 0 2 4 6 8 10 12 14
-5 0 5 10 15 20 25
Effluent Ammonia (mg.l-1)
Time (Hours)
Shower Gel Washing Powder bleach SDS phenol zinc sulphate
Figure 6-10 – effluent ammonia for all six toxins normalised to their respective controls
In contrast the washing powder dose reduced the effluent ammonia level compared to the control. Again this effect is most prominent 6 hours after the spot dosing, peaks at 12 hours and is still evident after 24 hours. In addition to this, the influent ammonia at the beginning of the trial was above the average recorded during baseline monitoring at 41.5mg.l-1. The washing powder has increased the pH of the system from 6.1 to 7 immediately after dosing (Section 6.3.4.4) which is the optimum operating pH for nitrifying bacteria (Tchobanoglous et al., 2003) and can be seen in the increase in ammonia removal in the test pots to 94 % compared to the control pots removal rate of 80
%.
The bleach dosing is the only trial where the toxin transits the system within the 24 hours and returns to close to steady state. However, as there is not a marked contrast between control and test pots for the bleach trial, this is most likely to be due to environmental factors rather than true inhibition of the
106 nitrifying bacteria. The test pots went from an effluent level of around 4mg.l-1 until an hour after dosing to 8mg.l-1 after 6 hours so a doubling of effluent ammonia when the pH was between 5.7 and 6.2. The control pots were also showing a higher than usual effluent ammonia of around 7mg.l-1 during most of the trial with a low pH of 6.5 but had decreased to 0.73mg.l-1 by the end of the 24 hour period when the pH was at 7, the optimum for the nitrifying bacteria as mentioned above.