WATER longer for the lowest and highest acetate concentrations. These differences may be due to different rates of utilisation of acetate by bacte ria, depending on whether they need to initiate enzyme production for its breakdown. Other reasons for differences in the lag phases may be due to interactions between the acetate, bacteria and other natural organic matter (NOM) present, w hich may cause inhibitory effects on bacterial growth. T he growth rate in the BRP method is dependen t on the substrate quality. As acetate was the maj or source of carbon in this work, the growth rates were very similar between O.1-0.3. T he small differences seen between waters could be due to the background NOM present. The Myponga and M organ samples had slightly higher than average growth rates. The unspiked samples fo r all the raw and treated waters showed much lower growth rates of between 0 .01 and 0 .2.
Raw a nd Treated Wat ers The raw waters were sampled from the River Murray and several reservoirs in metropolitan Adelaide. The reservoirs have different types of catchments: H ope Valley is located in a residential area, Myponga and Barossa in agricultural regions and Happy Valley in an area of mixed uses. All the treatment plants except Myponga are conventional treatment plants using alum as the primary coagulant (Myponga is a dissolved air flotation plant). T he Anstey H ill plant is the o nly plant to use prechlorinated raw water. T here was some variability in results between the raw and treated waters. Raw water did not always produce the largest increase in bio mass. This may be due to seasonal changes in water quality and the types of natural organic matter present. Raw water usually has a greater proportion of higher molecular weight organic compounds. During treatment these larger compounds may be broken down by oxidation into smaller, more assimilable compounds w hich are more readily biodegraded, promoting increased growth in the treated water. Treatment also alters the ratio of organics present in the finished water. The acetate spikes produced similar growth curves in the different waters. T here was an order of magnitude difference in biomass increase between the 2.9 and 0 .29 mg C/L acetate spikes. The variability obse rved in the differe nt water sources would be due to the different levels and types of o rganics present. In the treated waters Myponga, a highly coloured water, had the highest growth factors and Hope Valley the lowest. None of the water samples showed any secondary growth , suggesting that the AOC present in the raw and treated waters was biodegraded at the same time as the acetate. There was some variability of the bacterial response in the different waters which were spiked with acetate. Some waters had very similar lag phases, whilst fo r others the lag phase was
The work of Servais et al. (1987) was repeated using acetate. The additio n of inoculum without an acetate spike produced no additional growth, suggesting that all the AOC had been used (see Figure 3). The samples which were respiked and reinoculated during the plateau stage all showed further growth, with the exception of the highest acetate concentration of2.9 mg C / L in the treated water. For the samples with the highest acetate spike, growth appeared to reach a maxim um level of around 6 ppm SiO 2 turbidity. If the initial growth curve did not reach this level , when the sample was respiked and reinoculated the additional growth reached this level. A high purity water sample was not initially spiked with acetate, but was spiked with 2.9 mg C/L after growth reac hed the plateau phase. T he additional growth produced did not reach the same level and the total cell nu mber (TCN) cou nts were lower than the samples w hich we re originally spiked at this co ncentration. This suggests that there may have been so me other li miting nutrient. T o determine whether another nutrient became limiting at the highest acetate concentration, after the growth curve reached the plateau stage two duplicate samples were reinoculated and respiked and additional nutrient solu ti on was added to o n e of the samples. This ensured that no other nutrient wou ld be a limiting facto r. There appeared to be no real difference in growth between the two samples, with both having no additional growth once they reached the plateau stage (see Figure 4) . At the highest acetate spike 2.9 mg C / L no addi tional growth occu rred once the plateau was reached. T his suggests that at high acetate concentrations fu rther bacterial growth was in hibited by the buildu p of toxic metabolic waste products o r that the disso lved oxyge n had decreased to levels which wou ld not sustain the population.
Slopes and Regression Coefficients T here was a strong linear relationship in Milli Q water between the increase in turbidity and the acetate concentration. The slope fo r all the standard acetate additions up to 2.9 mg C/L was 1.56 with a regression coefficient of 0.959. The slope was then calculated fo r the standard acetate additions in Milli Q at the lower end of the range, w hich would correspond more closely to the range in natural water samples. The slope was m uch lower, 0.9 18, with a regression coefficie nt of0.923. Werner et al. (1992) calculated a slope of 2.3 when the change in DOC was plotted against the change in t urbidity fo r standard additions in Milli Q water. In the current work the slope was determined by plotting the change in turbidity against the acetate concentration, since the change in D OC was unable to be accurately determined. T his difference in the slope may also be due to differences in sample size, as Werner et
6
10
Slope= 1 .5
R = 0 .91 6 N = 1 40
5
/
d' u'i
/
E a.
.e
ct' en
4
E
.ea.
3
- - Acetate 2 .9 mg/L no nu trient
~
2
...... Aceta te 0.29 mg / L nutrient
I-
0.1
.~
.
""O
:.c :i I-
. r -Samples were rein oculated and respiked w ith acetate
,..--/'
¡-¡¡ -~
- - Acetate 2.9 mg/L nu trient
0 .01
:,
Acetate 0.29 mg/ L no nutrien1
<l -
1 '
/
: I
I
/ i .. . 10
20
30
40
50
60
70
80
Incub ation time (h rs)
Figure 4 Bacterial regrowth pote ntial f or Mill i Q water un der different conditio ns
0
I
0.0
I
0.5
1 .0
1 .5
2 .0
2.5
3.0
3 .5
Ace tate co n centratio n mg C/L
Figure 5 Standard acet ate additions for al l waters tested
WATER SEPTEMBER/ OCTOBER 1998
21