into the sewer during sampling. This highlights the necessity for care during sampling when dealing with heavy metals. Variations in Abundance of Slime
Although four industrial sites were originally selected for biofilm monitoring, two sites could not be sampled because there was insufficient slime. The reduction in slime may have been due to polJutants in discharges from adjacent industries causing the biofilm to slough or hindering regrowth of biofilm. Fortunately, a similar problem did not occur at either the electroplater or the tannery sites. One possible explanation for the sufficient slime at the electroplater site may be that this industry was surrounded by numerous residential dwellings. The higher organic loading at this site, contrasted to that at the discontinued sampling sites, probably assisted in the rapid regrowth of the slime on the sewer pipes adjacent to this industry. Consequently, the frequency at which sampling can be conducted, when this technique is employed, will be determined to a large extent by the rate of slime re-growth. This appears to be influenced by the proximity of the industry to residential dwellings or other sources of organic waste.
This observation may be explained by: The saturation of binding sites on the slime by chromium, thus preventing other metals being adsorbed. 2. The selective uptake of certain metals by the polymer, or by the slime microbial population. 1.
The Electroplater
The high aluminium concentration in slime from the electroplating site (Figure 2) was expected due to their industrial processes. Slime samples from the electroplater had the highest concentrations of lead, iron, copper and cadmium, although the latter was still a relatively low concentration compared to the other metals (the mean cadmium concentration over the three sampling dates was 42 µ.g ig dry weight of 'slime). The electroplater was surrounded by residential dweIJings which would provide a greater organic loading in the effluent passing this site compared with that at the tannery. Consequently, a more diverse and dynamic biofilm community could be present at the electroplating site. Also, residential effluent would be a sou~ce of a_wider spectrum of metals, and a continual supply of orgamc debns that would be available for complexing metals.
Metal Content from Different Sites
This survey identified large differences in metal content in slime between sites; this was particularly obvious for slime collected adjacent to industries known to employ certain metals in their industrial processes. Silver concentrations in slime were low at aIJ four sites over the sampling period, ranging from 2 to 30 µg i g dry weight of slime. This would be expected since silver is an expensive metal and losses would be avoided. Manganese was found in similar low concentrations (90 to 250 µg ig dry weight of slime) in slime from all sites. The Tannery
The high chromium concentration in tannery effluent was the reason for the highly significant (P < 0.01) site effect for chromium (Figure 1). The only other metals found in high concentrations in slime from this site were manganese (120 µgi g dry weight of slime) and iron (2,500 µgig dry weight of slime) which were in similar concentrations to those in slimes from the other sites studied. A grab sample of effluent from the tannery was taken in 1989 (Table 2). The metal that occurred in the highest concentrations was chromium, while iron and aluminium were next but in concentrations considerably lower than for chromium. In contrast the slime collected adjacent to the tannery had the lowest aluminium concentration of all slimes sampled. The manganese concentration in this slime was higher than the aluminium concentration, even though the reverse was found in the grab effluent. Table 1 Al, Cu, Fe in Slimes Sites:
I. Residential: Burnside, Blair Athol 2. Industrial: Tannery, Electroplater Metal Concentration (ug/ g dry wt of slime) Aluminium Copper
Iron
Collection date (1988)
22. 11
6.12
2976 *30040
8024
8264
22.11
6.12
8.11
22.11
5102
4800
3950
2234
2562
1676
1494
1816
477
498
517
5672
5086
3050
258
971
2006
II
16
13
2020
2036
4840
25380 31080 24320
2596
2376
2274
11260 10364
7800
Blair Athol Tannery Electroplater
8.11
8.11
SITE: Burnside
6.12
• This consid erably higher iron concent ration was suspected to be due to flecks of rust being knocked into sewer during the removal of the manhole cover.
Table 2 Metal concentration (mgl L) in grab sample of effluent from the tannery, collected on 1 March 1989. The grab sample was collected from the same site from which slime samples were taken in 1988. Al
Ca
Cr
Cu
1.62
192.6
211.4
0.14
Fe 7.59
Mg 185.1
Mn 0.35
Na
6491
Zn 0.81
Residential Sites
The main difference between the two residential sites was that predominantly household effluent entered the sewer in Blair Athol, whereas many light industries, eg service stations, food outlets, hairdressers, laundries, etc. discharged effluent into the sewer upstream from the Burnside sampling site. Klein et al. (1974) identified light industries, commonly found in residential areas, as significant contributors to metal contamination of effluents, which may explain the differences in metal ·concentration in slime between the two residential sites. Possible sources of metals found at the two residential sites in this survey are corrosion of plumbing materials and discharge of household chemicals. ' Just after World War II cast iron pipes were used for underfloor waste transfer in Adelaide in both residential and industrial establishments, while earthenware niping was used for external plumbing. Cast iron for piping was superseded by copper in the early 1950s, but the use of copper piping for waste transfer was discontinued due to increased cost. However, copper piping is still used for internal wastewater transport within multi-storey buildings. Currently, alJ water is transported within residences through copper piping and PVC piping is used extensively for both interior and exterior wastewater transfer. In older houses zinc galvanised pipes were also used for internal water transport. (R. Backshall, pers. comm.). Corrosion of piping may be a possible source of iron, zinc and copper, while zinc, tin, lead and cadmium may leach from PVC piping. These metals are used as stabilisers and additives_in the manufacture of synthetic rubber and PVC (Forstner and Whittman, 1983). Enzyme detergents have been found to contain trace amounts of iron, manganese, chromium, cobalt, zinc, strontium and barium (Angino et al, 1974). Furthermore, many organic compounds contain metal additives. For example gasoline contains tetraethyl lead; heavy duty oil contains lead; and lubricating oil is usually supplemented with molybdenum sulfide. These compounds may be added, albeit irregularly, to wastewater from residential dwellings and thus contribute to heavy metal contamination. Petrol was found on the sewer platform at Burnside on the last sampling date. However, no corresponding increase in lead concentration in the slime was detected. Aluminium and copper occurred in high concentrations in slime collected from residential sites. The mean concentration of aluminium at Burnside was approximately double that at Blair Athol. A possible reason for this may be differences in water supply between the two sites. The water supply to Burnside is unfiltered while the supply to Blair Athol is filtered. The untreated water supply to Burnside has
Continued on page 39 WATER February, 1990
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