Hidalgo County (Texas) Colonias Projects

These projects were conducted under the Economically Distressed Area Program (EDAP) of the Texas Water Development Board (TWDB). The prime contractor on these projects was Carter & Burgess, Inc. Venhuizen performed all work dealing with "innovative/alternative" management options as a subcontractor to Carter & Burgess. Venhuizen participated in three projects dealing with Hidalgo County colonias: the Rural Colonias Project, the scoping phase of the Regional Colonias Project, and the facility planning phase of the Regional Colonias Project.

The Rural Project entailed detailed system planning for 4 selected colonias, containing 22, 72, 83 and 84 lots. Venhuizen prepared system plans and generated cost estimates for the following options for each of these colonias:

1. Small-scale collective systems in which effluent from septic tanks on each lot flows through small diameter gravity sewers to multiple low-pressure-dosed (LPD) dispersal fields scattered around the perimeter of the colonia.
2. Small-scale collective systems in which effluent from septic tanks on each lot flows through small diameter gravity sewers to multiple sand filter treatment units, the effluent from which is piped to one or more points where it is disinfected and discharged into an irrigation drainage canal.
3. Large-scale collective systems in which effluent from septic tanks on each lot flows through small diameter gravity sewers to one large LPD dispersal field.
4. Large-scale collective systems in which effluent from septic tanks on each lot flows through small diameter gravity sewers to a single sand filter treatment plant, the effluent from which is disinfected and discharged into an irrigation drainage canal.
5. Large-scale collective systems in which effluent from septic tanks on each lot flows through small diameter gravity sewers to a single constructed wetland (gravel marsh) treatment plant, the effluent from which is disinfected and discharged into an irrigation drainage canal.

These detailed studies provided the basis for a "mainline" strategy for addressing other similarly situated colonias in the Regional Project. It was found that the estimated costs of the various strategies did not vary significantly enough to positively favor any one approach over the other. Considerations of managing a discharge permit vs. a soil dispersal system dictated that the LPD system options would be the favored course of action.

The estimated costs to serve existing houses using these decentralized concept options in these four colonias ranged from about $3,800 per house in the colonia with 64 existing units to about $6,000 per house in the colonia with 16 existing units. Cost per home at buildout ranged from about $3,300 to about $5,000. Estimated monthly O&M cost for the LPD systems were less than $10/home/month in all cases. Estimated O&M cost for the options with sand filter or wetland treatment plants ranged from about $12/home/month in the larger colonias to over $20/home/month in the smallest one. Estimated costs of hooking these colonias into a centralized collection and treatment system was over $11,000 per home in all cases, confirming that the decentralized concept is far less costly.

In the scoping phase of the Regional Project, approximately 125 colonias were evaluated for need and eligibility for participation in EDAP. Venhuizen assisted Carter & Burgess in evaluation of the costs of centralized sewerage service vs. decentralized management options like those studied in the Rural Project. Approximately 30 colonias were selected for inclusion in the facility planning phase of the Regional Project. Of those, 16 were determined to be definitely better suited to a decentralized management plan. Venhuizen was totally responsible for generating the facility plans for that group. These 16 colonias range in size from 18 to 114 lots. The total number of lots in this group is 737, yielding an average of 46 lots per colonia. Another 10 colonias, ranging in size from 17 to 96 lots (containing 478 total lots for an average of 48 lots per colonia), were evaluated for both conventional sewerage service and a decentralized management plan. Venhuizen generated the system plans and cost estimates for the decentralized option for those 10 colonias. The remainder of these colonias were considered to be close enough to an existing sewer line that centralized sewerage would be the least cost solution.

The results of Venhuizen's analysis indicated that these colonias could be serviced using the LPD system concept with a net present worth of construction cost in the range of $3,106-$4,594 per lot, including all planning, permitting, engineering, land acquisition and legal costs. The high cost was an "outlyer"-the next highest cost per lot was $4,118. The weighted average cost per lot was $3,527. The figure below shows the schematic system layout used to generate these cost estimates for one of these colonias. Layouts for all the colonias are similar.

O&M costs were also evaluated for the 16 lots in the first group and, based on the assumptions made, yielded costs in the range of $5.37 to $8.69 per lot per month. The weighted average among this group was $6.17 per lot per month. Cost factors included were inspections, electricity, an allowance for general maintenance, pump replacement (7-year intervals assumed), and pumping of septic tanks and dosing tanks.

As a comparison of these costs to the costs of hooking into a conventional, centralized collection and treatment system, the total cost for connecting 7 of these colonias was estimated at $2.08 million. The total cost of the decentralized systems for these seven colonias was estimated at $1.23 million, 59% of the cost of the centralized strategy. Since these colonias were situated much more favorably for connecting to the treatment plant than the 16 colonias in the first group, it is to be expected that there would be a much greater cost differential for that group.

These facility plans were completed in 1998 and are currently under review by the TWDB. The local sponsor of most of the decentralized projects, North Alamo Water Supply Corporation, has favorably received the decentralized management concept and has exhibited a readiness to build and manage these systems once these plans are accepted by TWDB. Toward that end, Venhuizen has worked with North Alamo staff to generate a management plan for system surveillance and maintenance.

(click the figure above to load a larger more detailed version ~37K)