October 2009 - South Tahoe Public Utility District

estimated 2028 population of 33,900 year round residents results in an estimated average
annual 2028 flow of 5.8 MGD as follows:
170 gal/cap/day x 33,900 cap = 5.76 MGD (round to 5.8 MGD)
The 5.8 MGD estimated annual recycled water flow from the District‟s wastewater treatment
facility equivalent to 6,498 AF/Yr is assumed to be the Harvey Place Reservoir daily inflow rate
in the year 2028.
The population estimate and subsequent recycled water flow quantity developed above should
be considered estimates only. Both the average annual flow per capita and the TRPA unit limit
per year were assumed to remain constant from the present to the year 2028. Each of these
factors can be expected to vary over time. The estimated recycled water flow quantity utilized in
this Master Plan should therefore be assumed to be accurate within a 20 percent range of the
value developed.
The recycled water flow for the year 2028 estimated by using Method 3 (based upon an
increasing population trend) is larger than that developed using Method 1 or Method 2 (based
on decreasing flow trends). It is believed that the decreasing flow data may be the result of a
trend toward vacation homes and away from primary residences in the Lake Tahoe area.
However, because population in the area is increasing, the more conservative value developed
using method 3 with a 5.8 MGD average flow is utilized in this Master Plan.
Table 5.2: WWTP Recycled Water Flow Method 3 Summary
Parameter Year 2028
Population *33,900
Annual Average Flow
Maximum Month Flow
*Adjusted to account for year-round residents
5.2 Recycled Water Quality
5.8 MGD
7.0 MGD
The District‟s WWTP is currently permitted for “secondary 23” recycled water. This means the
water has been oxidized and disinfected so that the median concentration of total coliform
bacteria does not exceed a Most Probable Number (MPN) of 23 per 100 milliliters (ml) and the
single day maximum does not exceed a MPN of 240 per 100 ml in any 30 day period. This is
explained further in the next section.
Secondary treated wastewater (recycled water) is pumped out of the Lake Tahoe basin into
Alpine County. Recycled water is stored in Harvey Place Reservoir. Recycled water from the
reservoir is used for agricultural reuse on several properties located between the reservoir and
the State line.
5.2.1 WWTP Recycled Water Quality
Recycled water quality data from the South Tahoe WWTP from the past ten years was
obtained. The following water quality parameters were examined for trends and used to project
the recycled water quality for the year 2028.
●
Biochemical Oxygen Demand (BOD)
South Tahoe Public Utility District
Recycled Water Facilities Master Plan - October 2009 Page 5-33