In June 2014, Comstock Construction recommended to the Spring Prairie Hutterite Colony that they retain Interstate Engineering to perform a design review of their wastewater stabilization ponds. At the time, the Colony had been working for two years on building their own centralized wastewater collection and treatment system. Construction had been performed based on drawings prepared by another consultant and approved by the Minnesota Pollution Control Agency (MPCA) with a permit specific deadline requiring completion by October 31, 2014.
The Colony was having difficulty constructing the two-cell, five-acre lagoon system and was not receiving satisfactory support from their current consultant. After a site visit and review of the design drawings, Interstate Engineering provided recommendations but declined the offer to take over the project. Interstate Engineering suggested that the Colony continue with their current engineer. In response, the Colony terminated their relationship with their current engineer and repeated their request to Interstate Engineering, which was accepted. Over the course of four months, Interstate Engineering performed a variety of tasks to assist the Colony. These included establishing benchmarks; geomembrane installation CQA; leak testing; and final recommendation to place in service.
Benchmarks had not been provided for the project by the original design engineer and the existing design information conflicted with itself. Upon commencement of work, Interstate Engineering surveyors established control for the site and performed a topographic survey of the pond as currently constructed. A three-dimensional model of the topographic survey and a modified pond design was prepared by Interstate Engineering staff. The design elevation of the pond was adjusted to minimize earthwork to reach the new Interstate Engineering design grades. The proposed design by Interstate Engineering was submitted to the MPCA and then approved for construction. The Colony retained Comstock Construction to do the fine grading work required to reach the approved pond grades. Interstate Engineering provided a three-dimensional model of the lagoon design so that Comstock Construction could use GPS guided equipment to complete the work.
Geomembrane Installation CQA
The MPCA requires full-time geomembrane installation construction observation. Prior to commencement of the geomembrane installation, Interstate Engineering reviewed the already contracted installer’s agreement with the Colony. Their proposed installation specifications did not comply with the MPCA requirements but met industry standards. Interstate Engineering worked through the differences with all parties to ensure that the MPCA would accept the work as installed. Upon commencement of work, Interstate Engineering furnished full-time construction observation for the 100-mil High-Density Polyethylene liner. Critical components of the work included trial welds; non-destructive and destructive sample testing. There were some failing trial welds, destructive and non-destructive test results. Interstate Engineering observed correction to failing welds to verify conformance with the project specifications. For example, the 100-mil liner is uncommon, and the installer had difficulties maintaining an air channel in their double seam fusion welding devices for non-destruct air testing. Upon completion of geomembrane installation, Interstate Engineering wrote a documentation letter requesting MPCA approval for the Colony to proceed with the following stage of the project – leak testing.
The MPCA requires leak testing for new lagoon cells to demonstrate performance-based statutory requirements of a maximum leak rate of 500 gallons per acre per day based on an 85% confidence interval. Interstate Engineering staff established a test plan and collected water level data in three barrel sets (as shown in the photo) for each pond for approximately three weeks. Based on the testing standards, the test passed one day after the minimum testing period elapsed.
Letter of Recommendation
Upon completion of the leak test, Interstate Engineering wrote a letter to the MPCA summarizing the results of the leak test and recommending approval and authorization to commence operation of the lagoons. The MPCA authorized the Colony to commence operations.
Interstate Engineering provided full-service engineering for the Spring Prairie Hutterite Colony. In this unique situation, Interstate Engineering worked on a tight schedule to assist in completing a project that was designed and permitted by a different company to meet a client’s needs.