In 1964, an agreement between the Town of Addison and the City of Farmers Branch allowed Addison to direct its sanitary sewer flows through Farmers Branch's sanitary sewer lines to the Trinity River Authority's (TRA) sewage treatment plant. The term of the agreement was 50 years, renewable for additional 50-year periods at Addison's option.
When Addison's westside began to develop, the Town asked Farmers Branch for additional flow capacity in the sanitary sewer lines to accommodate the development growth. Farmers Branch refused the request and the dispute resulted in a lawsuit between the two cities. Subsequently, Addison and Farmers Branch entered into discussions on an interim agreement that would allow Addison to continue to send its sewage flow to the Farmers Branch system and on the construction of a sewer tunnel that would convey the then current and future flows from both cities to the treatment plant. The agreements stipulated the amount of flow that Addison could send to the Farmers Branch system and the associated fee for that service that was based on 120% of the rate being charged to Farmers Branch by TRA. The agreement left in place one unmetered basin. For the right to send sewage from that basin through Farmers Branch's sanitary sewer system to TRA, Addison was required to pay Farmers Branch an amount equal to 80% of the amount of the customer accounts of water consumption, times the then current TRA rate. The last interim agreement expired on November 6, 1995. The sanitary sewer tunnel was completed in 1996.
Once the sewer tunnel went into service, Addison's westside sanitary sewer flow was directed into that facility except the flow in Basin I (then called the Beltwood portion of the Farmers Branch Creek Basin), which is bordered by Inwood Road to the east, Belt Line Road to the north, Beltwood Parkway to the west, and Addison's city limits to the south. An agreement for this portion of the basin was never executed but Addison has continued to pay Farmers Branch for the use of their system based on the formula as outlined in the last executed agreement.
In October 2017, Farmers Branch notified Addison that the unmetered flows in Basin I will need to be metered or a new agreement with Farmers Branch will be needed. On January 11, 2018, Addison requested to begin discussions with Farmers Branch on a new agreement to allow Addison’s sewage to flow through Farmers Branch’s system to Trinity River Authority.
On January 15, 2018, Farmers Branch notified Addison that the Town would need to work with TRA to install a metering station that would eliminate unmetered access points into their system. On March 24, 2018, Farmers Branch sent a letter to Addison requiring the Town to either divert the flow or install a TRA deduct meter by May 25th, at which time the line would be capped if either of these conditions are not met. Staff has been working with a consultant engineer to determine the best alternative for this improvement. Three alternatives have been considered:
| Alternative |
Description |
| A |
A new TRA approved metering station to measure sanitary sewer flows discharged into the Farmers Branch system |
| B |
A new gravity sewer collection line to re-route sewer flows away from the Farmers Branch system to adjacent Addison basin already connected to Trinity River Authority (TRA) meter station |
| C |
A new lift station and force main pipeline to re-route sewer flows away from the Farmers Branch system into an adjacent Addison sewer basin that is connected to a TRA meter station
|
Staff considered two options in Alternative A, A-1 and A-2. Alternative A-1 involved the installation of a metering station at the current location where Addison’s flow enters the Farmers Branch system. Alternative A-2 involved a new metering station that would discharge metered sewage directly into TRA’s system. A-2 would necessitate negotiating with Farmers Branch for the use of their public right-of-way to connect to the TRA system. Therefore, this option was rejected and was not included in the cost estimating or the Business Case Evaluation.
There were two options in Alternative B also. B-1 involved constructing 4,654 linear feet of gravity line via an open cut in Beltway Drive to connect to Addison’s existing sanitary sewer system at Midway Road. B-2 involved constructing 3,405 linear feet of gravity line via trenchless construction behind buildings through easements that would need to be negotiated with the private property owners. The construction cost of B-2 was estimated but the option was not included in the Business Case Evaluation discussed below due to the uncertainty of the cost and time to obtain easements from the private property owners.
There were two options in Alternative C, with the only difference being the ultimate location of the lift station and subsequent length of piping needed. The construction cost of both options were developed and both were included in the Business Case Evaluation.
The estimated cost for each alternative is as follows:
| Alternative |
Description |
Total Cost* |
| A-1 |
Construct metering station |
$1,377,388 |
| B-1 |
Gravity flow using Beltway Drive |
$1,617,513 |
| B-2 |
Gravity flow using easements to be acquired |
$1,466,714** |
| C |
Lift station, gravity flow line, and force main line |
$746,603-$772,764 |
*Cost includes capital costs and design of plans and specifications, contract administration, construction testing and inspection, estimated land acquisition, and associated costs.
**The cost of the easements on private property behind the buildings has not been estimated or included in this cost due to the uncertainty of the cost and time to obtain easements from the private property owners.
Staff considered several goals in analyzing the alternatives:
- The alternative that presents the best business case
- The alternative that redirects the flow out of the Farmers Branch system and into an existing TRA metered basin
- The alternative that has the lowest life-cycle costs for on-going operations, maintenance, and refurbishment
- The alternative that allows for future growth with no permission needed from other jurisdictions
Staff conducted a Business Case Evaluation that showed that Alternative B-1 is the most cost effective alternative, even though the initial cost is higher than the other alternatives. Alternative B-1 has the lowest life-cycle costs related to on-going operations, maintenance, and refurbishment costs. A detailed spreadsheet of the Business Case Evaluation is included as an attachment to this item.
Based on this information, staff recommends Alternative B-1, the gravity flow using Beltway Drive be designed and constructed. If so directed, staff will bring an engineering design contract for the design of Alternative B-1 to Council for consideration on May 22, 2018. |