OPTION #1: Engineering, planning, quality control
OWNER: Clinton Public Works Department, Water Pollution Control Division
Three Iowa cities used underwater expertise to clear a potential regulatory hurdle while developing a regional wastewater treatment plan.
Despite five decades of upgrades that increased capacity to 10 mgd, Clinton's secondary activated-sludge treatment plant consistently failed to meet requirements for discharging to the Mississippi River. Seeing an opportunity to resolve compliance issues for several communities, the Iowa Department of Natural Resources encouraged the city to build a plant large enough to treat flows from the neighboring cities of Camanche and Low Moor as well. Expected to be on line by 2013, the 12-mgd biological nutrient-removal activated-sludge facility will accommodate a 60% increase in organic loads.
The state contributed to this goal with a grant to help Camanche install a pipeline to Clinton's system. Just one hitch: To qualify for the funding, the connection would have to be made before Clinton's new plant was operational. If that happened, the volume of noncompliant effluent flowing into the Mississippi River would increase.
When regulators sought an interim solution for minimizing violations, Stanley Consultants Inc. proposed lowering discharge requirements by enlarging the old plant's mixing zone. Extending the plant's outfall 300 feet into the river would deposit effluent beyond a major tributary that empties into the river immediately downstream of the outfall, giving the plant credits to apply toward its permit.
A nifty solution. But in addition to fast-tracking the necessary permits, engineers also had to figure out how to keep the extension out of the river's navigation channel while avoiding a high-pressure gas pipeline buried in the riverbed and high-voltage power lines swaying overhead.
Stanley Consultants subcontracted Martin & Whitacre Surveyors of Muscatine, Iowa, to survey the riverbed. In addition to those results, engineers aligned the extension so it would discharge downstream of the tributary's confluence with the river, avoid barge mooring facilities, and avoid the downstream edge of an upstream mussel bed.
Civil Constructors Inc. of East Moline, Ill., was the construction contractor. The 36-inch-diameter, fusion-welded, high-density polyethylene extension was assembled on a barge, lowered into the excavated riverbed, installed and connected into an existing outfall sewer manhole structure in the riverbank, then covered with rip rap and articulated concrete mat.
In addition to planning and design, Stanley Consultants provided resident engineering services and two dives once the extension was installed: One after the pipe was seated to confirm placement location, continuity, embedment depth, proper bedding stone, and adherence to the design plans; the second during the placement of cover stone and the final covering of articulated block to confirm proper placement and coverage of the mats and confirm completion of the underwater tasks.
The project was conceived in the late fall of 2008. Planning, design, permitting, and construction occurred in 2009.