Technical Reports Document Library

Note:  This report is included for its historical value and may have been superseded by more recent studies.

This study was authorized in April of 1969 by the City of San Antonio, the San Antonio City Public Service Board, the San Antonio City Water Board, the Bexar Metropolitan Water District, the San Antonio River Authority, and the Edwards Underground Water District. It gives consideration to the following principal objectives:

a. Use where practical of treated wastewater for applications which will tend to minimize future withdrawals from the Edwards Underground Reservoir.

b. Effective development of the Medina River watershed for recharge of the Edwards Underground Reservoir and as a supplemental source of surface water supply for San Antonio.

c. Reclamation of Mitchell Lake for esthetic, recreational and other potential uses.

d. Possible gains in recreational use at Lakes Braunig and Calaveras because of improved water quality and at Lake Medina from maintenance of higher water levels.

Included herein are estimates of future wastewater volumes and of the requirements which might be met through use of reclaimed wastewater in lieu of ground water pumpage. The hydrology of the Medina River Basin is outlined in some detail to show the influence of Lake Medina on recharge of the Edwards Underground Reservoir and the amount of surface water which can be obtained at the Applewhite Reservoir site .

Attention is given to the character of wastewater treatment which would be required to produce water of suitable quality for the contemplated applications. Finally, economic analyses are presented relative to costs, benefits, and possible methods of financing.

As results of the investigation were reviewed with representatives of the sponsoring organizations, it was found desirable to add further items to the scope of the work. In July of 1970, it was agreed that the study would include discussion of a gravity-flow interconnection between Lakes Mitchell, Braunig and Calaveras. In October 1970, it was decided to give consideration to possible irrigation along the south side of the San Antonio River in southeastern Bexar County and northwestern Wilson County. The first of these items was included because of its potential relevance to power plant operation at the lakes and to over-all quality of waters in the lakes and the San Antonio River. Irrigation to the southeast of San Antonio, although it would not reduce withdrawals from the Edwards Underground Reservoir, is an apparent alternative use for the reclaimed wastewater and was added so that it might be compared with the various applications which would affect the availability of  water from the Edwards limestone.

CONCLUSIONS AND RECOMMENDATIONS

a. San Antonio’s wastewater reclamation plants are currently producing approximately 100,000 acre-feet per year of water that can be re-used for some purposes, and the amount is expected to increase to 173,000 acre-feet per year by the end of the century.

b. The two most promising uses for reclaimed wastewater in the San Antonio area are irrigation and pow1er plant cooling. At the present time, 16,200 acre-feet per year have been committed to existing obligations for irrigation in the vicinity of Mitchell Lake, and 72,000 acre-feet per year are covered by permits for diversions to Lake Braunig and Lake Calaveras.

c. Once the Braunig and Calaveras Power plants are developed to full capacity, water requirements to replace evaporative losses and maintain suitable chemical concentrations in the cooling lakes will be more than the present diversion permits. The estimated peak requirements are 24,000 acre-feet per year at Lake Braunig and 69,800 acre-feet per year at Lake Calaveras. Their existing permits are for 12,000 acre-feet per year and 60,000 acre-feet per year, respectively.

d. In general, power plant cooling use offers greater over-all benefits than irrigation, especially in terms of ground water conservation and recreation. Where it is necessary to choose between the two, first preference should be given to the power plant needs.

e. The total of existing obligations and other potential uses will exceed the available supply of reclaimed wastewater for the next 20 years or longer. There probably will not be sufficient uncommitted wastewater for new 1arge-scale irritigation until 1990 or after. The proposed irrigation operations west of San Antonio, which would require from 25,000 to 40,000 acre-feet per year, are thus long-term considerations rather than prospects for the near future. A lesser volume (9,200 acre-feet per year) of new irrigation southeast of San Antonio could be supplied now if backed by a moderate amount of regulating storage.

f. Eventual use of reclaimed wastewater to meet the irrigation requirements of Bexar-Medina-Atascosa Counties Water Improvement District No. 1 in lieu of the supply from Lake Medina would gain an average of 15,700 acre-feet per year in increased recharge and yield from the Medina River. The amount of reclaimed water needed would be in the range of 25,000 to 35,000 acre-feet per year, plus an estimated 20% additional to replace losses in the conveyance system. The corresponding unit expenditure per 1,000 gallons of increased recharge and yield would range from 21.5¢ to 26.0¢ at present-day cost levels. Operation of Lake Medina for recharge instead of irrigation would result in higher water levels in that reservoir part of the time; however, the recreational improvement would not be sufficient to offset the inherently high costs of delivering the reclaimed water to the Medina canal. Use of reclaimed wastewater for irrigation in the Bexar-Medina-Atascosa District does not appear to be economically justified.

g. Use of reclaimed wastewater for irrigation of other lands along the Medina River in western Bexar and eastern Medina Counties would reduce the long-range ground water pumpage in that area by as much as 40,000 acre-feet per year for an estimated unit cost of 11.2¢ per 1,000 gallons of ground water conserved at today’s cost levels. Use of the reclaimed water for large-scale irrigation of such lands would be economically justified on that basis.

h. Approximately 4,000 acres of land along the south bank of the San Antonio River in southeastern Bexar County and northwestern Wilson County could be supplied with reclaimed wastewater for irrigation. The supply would be dependable only if supported by some 3,300 to 3,700 acre-feet of regulating storage not already committed to other purposes. Mitchell Lake could serve this need if not otherwise utilized. The estimated cost of delivering the reclaimed water to this area is $32.50 per year per acre.

i. Sites for additional power plant cooling lakes near San Antonio are limited, and the only likely prospect appears to be at Mitchell Lake. It is feasible to raise Mitchell Lake 12 feet, which would provide enough surface area to support a generating plant of about 1,400 megawatts capacity. The estimated capital cost to raise the water surface 12 feet is $1.58 million. If Mitchell Lake is to be considered seriously as a power plant site, it must be available in the immediate future. After about 1978, a lake of that size may well be too small to accommodate the large generating units projected to be needed by that time.

j. Use of Mitchell Lake for something other than its present purpose would require construction of new facilities for disposal of waste activated sludge and primary digester supernatant at the Rilling Road plants. The estimated capital cost of providing such facilities is $3.5 million. The added unit cost of sewage treatment at the Rilling Road plants due to adoption of a new system for waste activated sludge and digester supernatant could be expected to be between $15 and $20 per million gallons treated, including debt service on the capital investment.