OAXACA, MEXICO

Version 2.0:  September 10, 2004

• Design features: The pond is enclosed within earthen dike. It has inlet and outlet works. Design is based on a balanced cut and fill so that the excavated material is used in dike construction. Outside slopes of dike are 3:1; inside slopes are 2:1. The pond has three cells in series and parallel, for a total of nine cells. The cells in series provide monitoring and control. The cells in parallel enable plug flow and provide a better effluent. The parallel cells also serve the purpose of maintenance, since one parallel track can be taken out of service temporarily. The cells will be separated among themselves with brick walls, with specific provision for the maintenance of the flow gradient. The flow gradient should not be less than 1% and not more than 2%. The flow gradient will be determined based on the local topography and the physical orientation of the pond with respect to the wind direction.

• Depth: Aerobic ponds should not be too shallow or too deep. If they are too shallow, they may be destroyed by emergent vegetation. If they are too deep, light penetration may be impaired and the lower layer may become anaerobic. A depth of 1 m is not too shallow nor too deep. Assume a depth = 1.2 m.

• Mixing: Mixing of pond contents contributes to the distribution of dissolved oxygen throughout the profile. Mixing is primarily by wind action and secondarily by temperature gradients. The maximum effect of wind action can be obtained when there is an unobstructed wind path of 100 to 200 m. Ponds should be designed with a longitudinal orientation parallel to the prevailing winds. The direction of the prevailing winds in Tlaxiaco will have to be determined experimentally, since there is not wind direction data available. Trees provide a barrier to wind action, so care should be taken to minimize the presence of large trees upwind of the flow path.

• Design procedure: Use the Wilhelm-Werner equation  for chemical reactor design (Reed, S. C., R. W. Crites, and E. J. Middlebrooks. 1995. Natural Systems for Waste Management and Treatment, McGraw-Hill, Inc., New York).

• Sewage load count: To determine the sewage load, the APASZU 96 map of the city of Tlaxiaco (December 1996) was provided by the Office of Water and Sewage of Tlaxiaco. The city was divided into five regions:

  1. Collector A, the main sewage collector for Tlaxiaco, including San Nicolas,

  2. Collector B, the secondary collector for Tlaxiaco,

  3. San Diego, to the North,

  4. San Bartolome and San Pedro, to the Southeast, and

  5. San Sebastian, to the West.

  The 1996 map has three categories of connections to the municipal sewage system: (1) active, (2) inactive, and (3) undeveloped. The sewage load was estimated using the following assumptions:

  • All inactive and undeveloped connections are eventually converted to active.

  • Only 50% of San Diego (North) and San Bartolome and San Pedro (Southeast) drain into the municipal system. The remaining areas (predominantly rural) will use latrines.

  • 100% of collectors A, B, and San Sebastian region will drain into the municipal sewage system.

  The count is shown in the following table. The total number of connections is calculated to be 2459. Therefore, a rounded value of 2500 is adopted for design.

  Region	Active	Inactive	Undeveloped	Total	Connected (design)	Adopted
  Collector A	479	112	51	642	642
  Collector B	127	20	23	170	170
  San Diego	276	164	166	606	303
  San Bartolome and San Pedro	392	251	274	917	459
  San Sebastian	515	170	200	885	885
  Total	1789	717	714	3220	2459	2500

• Load calculation: Assume 0.15 m³/day/person of water consumption in Tlaxiaco; assume factor 0.6 of water-to-sewage conversion; then, 0.09 m³/day/person of sewage load. The pond will be designed for 2,500 families draining to the municipal sewer system. With an estimate of 4 persons per family, this results in 10,000 persons total, or 900 m³/day sewage load; or 0.0104 m³/sec. Assume a sewage load of 0.01 m³/sec for design purposes.

• Sewage concentration calculation: Assume a load of 30 grams/person/day of dry fecal matter, applicable to developing countries such as Mexico. Then, the sewage concentration will be:

  30 gr/person/day X 10 000 persons X 1000 mg/gr / (900 m³/day X 1000 liter/m³) = 333 mg/liter (ppm).

  Assume 350 ppm for design purposes.

• Design example:

  The pond has length L, width W, and depth d. Assume three cells in series and three cells in parallel, for a total of nine cells.

  The mean temperature of the coldest month (January) is T= 13 ^oC. Assume that the design dispersion number D= 0.1 (plug flow), subject to verification. Assume a hydraulic retention time t= 20 days. Therefore, the BOD removal efficiency is 76.57%.

  With design Q= 0.01 m³/sec, the volume of the pond is V= LWd= 0.01 m³/sec X 20 days X 86400 sec/day = 17,280 m³. With a design depth d= 1.2 m, then, the area of the oxidation pond is LW= 14,400 m².

  Assume a width W= 90 m. Then, L= 160 m. Assume n= 3 cells in parallel. Therefore each cell has a width w= W/n= 30 m.

  Calculation of D:

  D= 0.184 [tv(w + 2d)]^0.489w^1.511 (Ld)^-1.489

  v= kinematic viscosity (m²/day)

  At 13 ^oC, v = 1.21 centistokes = 0.0121 stoke = 0.0121 cm²/sec = 0.0121 cm²/sec X 0.0001 m²/cm² X 86400 sec/day=
  v= 0.1045 m²/day

  D = 0.184 [20 X 0.1045 (30 + 2.4)]^0.489 (30)^1.511 (160 X 1.2)^-1.489 =

  D = 0.184 (67.716)^0.489 (30)^1.511 (192)^-1.489 =

  D = (0.184) X (7.856) X (170.58) X (0.0003982) = 0.0982

  Since calculated D= 0.0982 is sufficiently close to design D= 0.1, the effective BOD removal for t= 20 days is 76.6% = 77% For influent BOD= 350 ppm, the effluent BOD from the oxidation pond is = 350 X (1 - 0.77) = 80 ppm

  Remarks:

  The design pond dimensions are L= 162 m, d= 1.2 m, and W = 90 m, with n= 3 parallel cells of w= 30 m each, and three cells in series of l= 54 m each. The actual BOD removal is 77% and the effluent BOD is 80 ppm.

• Calculation of volume of dike: Assume depth of dike = 1.8 m; top width= 3 m; upstream slope 2:1; downstream slope 3:1; total area 13.5 m³/m. Length of dike = 520 m. Total volume of dike= 7014 m³. Unit cost 40 Mexican pesos per cubic meter. Therefore, cost is $ 280,560 (Mexican pesos).

  Depth of excavation = 7014 m³ / 14,580 m² = 0.48 m.
  Removal of surface layer of 0.15 m.
  Total depth of excavation: 0.63 m.

  Estimated project cost = 2 x 280,560 = $ 561,120 (U.S. $ 62,347).