Environ. Sci. Technol. 2008, 42, 4280–4284
Nitrification in Premise Plumbing: Role of Phosphate, pH and Pipe Corrosion YAN ZHANG, ALLIAN GRIFFIN, AND MARC EDWARDS* 418 Durham Hall, Civil and Environmental Engineering Department Virginia Tech, Blacksburg, Virginia 24061
Received October 1, 2007. Revised manuscript received February 18, 2008. Accepted February 20, 2008.
Nitrification in PVC premise plumbing is a weak function of pH over the range 6.5–8.5 and is insensitive to phosphate concentrations 5–1000 ppb. Lead pipe enhanced nitrification relative to PVC, consistent with expectations that nitrifiers could benefit from ammonia recycled from nitrate via lead corrosion. Relatively new copper pipe (95% confidence. Lead Pipes. In lead pipes, phosphorus levels had a more dramatic effect on lead release than did pH changes. Regardless of inhibitor dose, total and soluble lead release did not increase when the initial pH was decreased from 8.5 to 7.5, but soluble lead release increased markedly (Supporting Information-Table S-3) when initial pH was reduced to 6.5–7.0. The effects of pH and orthophosphate on soluble lead were roughly consistent with solubility model predictions (26–28). Soluble and total lead release were reduced by addition of 1000 ppb orthophosphate (Supporting Information-Table S-3), but was less significantly impacted by 60 ppb orthophosphate compared to the lowest dose of 5 ppb, consistent with earlier work and solubility models (26–28). Combined Effect of pH and Phosphate Levels on Nitrifier Activity. PVC Pipes. Reported optimal pH for nitrifier growth generally falls between 7.5 and 8 (29). In PVC pipes average nitrifier MPN was 0.5–1 log lower at pH 6.5 than at pH 8 (Table 2), however, this decrease was not statistically different at 95% confidence (Supporting Information-Table S-3). The decrease in pH did not reduce ammonia loss in PVC pipes, since complete ammonia conversion to nitrite and nitrate occurred even at pH 6.5. Thus, nitrifiers can remain active in PVC premise plumbing at much lower pH ranges than have been previously reported (29). At phosphorus levels below about 50 ppb-P, it has been reported that nitrification was inhibited via nutrient limitation (25). However, in this study using PVC pipe, nitrifier MPN (Table 2) and nitrifier activity (indicated by ammonia loss) was very high even at 5 and 60 ppb-P levels. This might be due to the higher levels of phosphorus used (1000 ppp-P) in the 6 month inoculation period before the phosphorus level in the medium was reduced to 5 or 60 ppb. The high cell phosphate content in nitrifier biofilm (25) or pipe surface deposits containing phosphate might serve as reservoirs for bioavailable phosphorus after the level introduced to the pipe was reduced. The effects of high cell phosphate content has also been reported to be more significant at lower ammonia/nitrite levels (25). Copper Pipes. In copper pipes, nitrifiers were only detected at pH 8 (Table 2) with 60 or 1000 ppb phosphate. MPN indexes were 2 orders of magnitude higher at 1000 ppb versus 60 ppb phosphate (Table 2). Both trends were consistent with expectations for lower copper toxicity to nitrifiers in the presence of phosphate and higher pH. Average ammonia loss was less than 10% at all pH and phosphorus levels, illustrating the role of copper pipe in preventing nitrification in premise plumbing relative to PCV or lead.
TABLE 1. Metal Release in Brass Pipes. Note: Each Data Reported Was the Average of at Least Four Weeks Testing pH
total Cu, ppb
soluble Cu, ppb
total Zn, ppb
phosphorus-ppb
5
60
1000
5
60
1000
5
60
1000
6.5 7 7.5 8 8.6
74 70 83 75 92
91 81 72 76 80
74 71 83 70 69
52 53 47 47 35
63 47 33 37 34
53 47 35 32 20
12168 6591 4623 3420 2336
10966 6108 2561 3995 1860
10250 6691 4528 3941 2845
TABLE 2. Logarithm of nitrifier MPN at different phosphorus and pH levels in different pipe materials pH
PVC copper brass lead
P-ppb
8
7.5
7
6.5
5 60 1000 5 60 1000 5 60 1000 5 60 1000
5.5 5.5 5.7
