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NRC (1993) refers to secondary treatment plants as "biological" plants. BOD loads are converted to units of labile organic carbon by assuming 1 mole of organic carbon oxidized for every mole of O2 consumed. Costs are based on averages for facilities in the United States, assuming an 8% interest rate, 20 year design period, and facilities designed to handle 72.6 x 103m3 d-1 of effluent. Land costs are not included. "Operating" costs include maintenance and operating costs. Note that costs are for cumulative level of treatment; secondary treatment includes primary treatment, and nutrient-reduction treatment includes both secondary and primary treatment.

NRC (1993) refers to secondary treatment plants as "biological" plants. BOD loads are converted to units of labile organic carbon by assuming 1 mole of organic carbon oxidized for every mole of O2 consumed. Costs are based on averages for facilities in the United States, assuming an 8% interest rate, 20 year design period, and facilities designed to handle 72.6 x 103m3 d-1 of effluent. Land costs are not included. "Operating" costs include maintenance and operating costs. Note that costs are for cumulative level of treatment; secondary treatment includes primary treatment, and nutrient-reduction treatment includes both secondary and primary treatment.

a ban (as of 1973) in the use of phosphates in detergents (Hetling et al., 2003). Considering only the effects of wastewater treatment changes, and using data for average effluent streams in the United States (Table 10.4), we estimate that P loadings to the Hudson estuary from wastewater sources would have decreased from an estimated 5 x 103 tons P y-1 in the early 1970s to 3.7 x 103 tons P y-1 in the mid 1990s. The effect of the ban on phosphates in detergents was probably greater (Clark et al., 1992; Hetling et al., 2003; Brosnan et al., Chapter 23, this volume). If we take the estimates of Hetling et al. (2003) and Brosnan et al. (Chapter 23, this volume) and scale them to the smaller effluent released from the smaller watershed we are considering, an estimated 8.5 x 103 tons P y-1 were loaded to the Hudson River Estuary from wastewater treatment plants in the early 1970s (Table 10.2). This large decrease in P in the estuary is broadly consistent with a three-fold reduction in phosphorus loadings estimated by Clark et al. (1992) based on observations of soluble reactive phosphorus (SRP) in the estuary over time, the assumption that SRP is conservative within the estuary, and a transport model. Note, however, that the assumption that SRP is conservative in the Hudson estuary (Clark et al., 1992) may be less valid in the 1990s than in the 1970s, as increased GPP wouldhave assimilated more SRP in the 1990s, and higher oxygen concentrations in the water column may have increased the phosphate adsorptive capacity of bottom sediments as well (Howarth et al., 1995). Considering all sources, we estimate that P loading to the Hudson

River Estuary decreased by a factor of two, from 9.6 x 103 tonsPy-1 to 4.8 x 103 tons Py-1, between the early 1970s and the mid 1990s (Table 10.2).

The upstream tributary sources of nitrogen have probably changed rather little in the Hudson basin since the 1970s (Jaworski, Howarth, and Hetling, 1997). We are aware of no data that would allow us to estimate how the upstream tributary inputs of phosphorus have changed since 1970. In any event, it seems likely that the wastewater sources were a greater percentage of the total nutrient input in the 1970s than in the 1990s.

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