October 2009 - South Tahoe Public Utility District

5.2.2 Trend Analysis
The BOD, NO 3 -N, TKN and TP concentrations show a slight increasing trend over the past ten
years (Figures 5.2.1(a-d)). Using these trends to project future average recycled water
concentrations gives a BOD concentration 8.2 mg/L, an NO 3 -N concentration of 0.49 mg/L, a
TKN concentration of 31 mg/L, and a TP concentration of 4.6 mg/L in the year 2028. At the
projected 2028 flow of 5.8 MGD, these concentrations correspond with a BOD load of 397
lb/day, an NO 3 -N load of 23.7 lb/day, a TKN load of 1,501 lb/day and a TP load of 223 lb/day.
BOD, NO 3 -N, TKN and TP are operationally controlled parameters and recycled water
concentrations can be decreased when necessary by applying additional oxygen, bringing
unused aeration basins on line, or modifying aeration basin operational parameters.
The SS concentration shows a slight decreasing trend over the last 10 years, while the TDS,
and Cl concentrations show a slight increasing trend (Figure 5.2.1(e), 5.2.1(f), and 5.2.1(g)).
Using these trends, an SS average recycled water concentration of 1.5 mg/L, a TDS average
recycled water concentration of 305 mg/L, and a Cl average recycled water concentration of 60
mg/l in the year 2028 are projected. At the projected 2028 flow of 5.8 MGD, these
concentrations correspond with a SS load of 73 lb/day, a TDS load of 14,764 lb/day, and a Cl
load of 2,904 lb/day. These concentrations are not affected by additional biological treatment.
Additionally, concentrations of TDS and Cl are expected to rise due to water conservation
efforts.
Several data anomalies are evident in Figure 5.2.1 (b). No explanation is evident as to why the
data anomalies exist. The trend line shown does include the anomaly data input.
5.2.3 Projected Water Quality
The projected water quality for the District‟s recycled water is summarized in Table 5.3.
Appendix B contains the technical memorandum detailing these projections.
Table 5.3: Existing and Projected Recycled Water Quantity and Quality
Parameter Year 2028
Population 33,900*
Flow
Annual Average
Peak Month
5.8 MGD
7.0 MGD
BOD 8.2 mg/L 397 lb/day
TKN 31 mg/L 1,501 lb/day
NO3-N 0.49 mg/L 23.7 lb/day
SS 1.5 mg/L 73 lb/day
TDS 305 mg/L 14,764 lb/day
Chloride (Cl) 60 mg/L 2,904 lb/day
TP 4.6 mg/L 223 lb/day
*Adjusted for year-round residents
Lands receiving recycled water may be either hydraulically limited rather or nutrient limited and
preparation of Nutrient Management Plans should determine which is the case.
South Tahoe Public Utility District
Recycled Water Facilities Master Plan - October 2009 Page 5-38