water treatment 78
UVT (%)
77 76
Presentedatat presented
Pre-Media Filter Chemical Dosing
Free Cl mg/L
refereed paper
Total Cl mg/L
Linear (Free Cl mg/L)
Linear (Total Cl mg/L)
77
75
Chlorine Residual (mg/L).
Turbidity (NTU).
UVT (%)
Jar testing was carried 74 76.5 73 out to improve our 72 understanding of 76 0 1 2 5 10 25 the impact on water CIP return rate (mL/L) 75.5 quality of pre-media Figure 2. Impact of acid CIP waste return filter chlorine and alum rate on UVT. 75 doses and contact times. The results 0.5 74.5 confirmed earlier 0.4 observations that 0.3 74 0.2 0 2 4 6 8 10 12 14 16 18 increased chlorine 0.1 Chlorine Residual (mg/L) and alum doses of 0 up to 20mg/L and 0 1 2 5 10 25 Figure 5. Impact of chlorine residual on UVT. CIP return rate (mL/L) 15mg/L respectively caused no significant Methanol Dosing Figure 3. Impact of acid CIP waste return improvement in performance. In fact, rate on turbidity. At the time UVT problems were increasing chlorine and alum doses encountered, methanol was being The observed increase in filtrate in excess of 10mg/L and 7.5mg/L overdosed to the bioreactor due to turbidity is most likely caused by the respectively caused no appreciable insufficient turn-down of the dosing interaction of the cleaning chemicals improvement in UVT, turbidity or colour. pumps. It was unlikely that such a with the filter media, which is difficult Jar testing was also used to assess the readily biodegradable substrate would to replicate in jar tests. By minimising impact of contact time on water quality. not be entirely consumed within the the CIP waste return rate, detrimental There was no easy way to increase bioreactor, but on the chance that the impacts are avoided. the contact time; however, it may have overdosing was to such an extent as The occurrence of CIP waste chemical provided clues as to the nature of the to make breakthrough possible, a jar returns at discrete intervals means that problem at hand, or provided an option test was carried out in which a range of if they were causing a reduction in UVT, for a longer-term solution. Contact times methanol doses were applied to effluent. it would be apparent in the UVT trend at of 90 mins and 180 mins were assessed The methanol had no impact on the UVT. similar intervals. From observation of the for comparison against the design UVT trend it was concluded the CIP waste contact time of 30 mins. It was found that Pre-Ultra Filter Chlorine Dosing return was not causing a reduction in UVT. increasing the contact time increased the Chlorine is dosed to the UF feed to reduce UVT very marginally at high chlorine doses, the occurrence of biological fouling on the but not sufficiently to warrant further membranes. The dose setpoint is 2mg/L; investigation. however, persistent spikes of up to 10mg/L free chlorine had been recorded due to It was also considered whether the imperfections in the dosing set-up. Jar chlorine dose was ineffective due to either tests were carried out in which the UVT under-strength chlorine solution or high was measured for a variety of chlorine chlorine demand due to sludge builddoses. The method required refinement up in the contact chamber. The chlorine when it was observed that spiking chlorine solution was tested and found to be within as a stock solution caused the UVT to specification. The contact chamber drain increase due to dilution with the high was opened, which discharged a small UVT deionised water. To overcome this quantity of sludge, but then ran clear. Grab problem a micropipette was used to samples were analysed at the start and end of the contact tank and in the media dose neat sodium hypochlorite. CIP and UF strainer. filter filtrate. It was found that the chlorine The results, presented in Figure 5, residual remained indicate that the spikes of 10mg/L would Free Chlorine Total Chlorine constant through reduce the UVT by an additional 1–1.5% the contact tank 25 compared to the normal chlorine residual then was almost of 2mg/L. This was not enough to explain completely consumed 20 the reduction in UVT observed, and the by the media filters, as frequency of free chlorine spikes was not shown in Figure 4. 15 mirrored by reductions of UVT of a similar Hence it was frequency, but it was accepted that this 10 concluded that there was an issue that could contribute to was not abnormal reduced UVT and should be resolved. 5 chlorine demand Media Filter Operation through the contact 0 tank, but the chlorine An unexpected but key observation was Dosed Start Contact End Contact Filtrate demand through made that revealed the path to a solution. Tank Tank the media filter was All four filter beds were being operated higher than expected. simultaneously until one required Figure 4. Fate of pre-media filter chlorine dose.
88 AUGUST 2011 water
technical features