Design and Start-up of Constructed Wetlands

For Cle Elum, Washington

Ronald W. Crites

Natural Systems Service Leader

Brown and Caldwell

2701 Prospect Park Drive

Rancho Cordova, CA 95670

(916) 444-0123

Rcrites@brwncald.com

Presented at the 68^th Annual PNPCA Conference

Pacific Northwest Pollution Control Association

Sunriver, Oregon

October 29, 2001

Design and Start-up of Constructed Wetlands

For Cle Elum, Washington

Ronald W. Crites

Natural Systems Service Leader

Brown and Caldwell

Abstract

Constructed wetlands have considerable potential for use in upgrading treatment pond systems. In 1998 the pond system used by the City of Cle Elum had a history of violating its NPDES permit.  Flows and loadings to its treatment plant were projected to grow significantly in the next twenty years, yet the treatment plant was already at capacity.  Planning efforts were conducted to evaluate the relative merits of several alternative long-range plans to accommodate increased wastewater flows and loads due to population growth in the area. Conversion of the third lagoon to a constructed wetland was chosen as an interim upgrade to handle increased treatment capacity and more stringent discharge requirements. 

The new revised permit required BOD and TSS concentrations not to exceed 30 mg/L and a minimum removal of 85 percent of the incoming BOD and TSS loads.  The treatment efficiency was the driving design parameter because of high infiltration/inflow (I/I) from the spring snow melt that creates a dilute waste stream requiring effluent BOD concentrations of 14 mg/L. Actual performance for BOD and TSS removal has ranged from 94 to 98 percent.

The wetland consists of a series of three planted zones separated by two open zones.  The planted zones, totaling 68 percent of the total area, are 1.5 feet deep and are planted with hardstem bulrush, Scripus acutus, a common native wetland plant.  The open zones are 3 feet deep, which prevents growth of wetland plants.  The planted zones allow for denitrification and BOD removal.  The open zones reduce short-circuiting and allow natural aeration to increase dissolved oxygen.

Background

The use of free water surface (FWS) constructed wetlands has ranged from achieving secondary treatment, to polishing of secondary effluent, to providing wildlife habitat and reuse of the water. High removals of BOD and TSS can be expected from FWS wetlands, along with significant removals of nitrogen, metals, trace organics, and pathogens.

There are two principal types of constructed wetlands – free water surface (FWS) and subsurface flow (SF). The design criteria FWS treatment wetlands are summarized in Table 1.  The limiting design criteria are often BOD and TSS removal for small municipal systems. Other parameters for design consideration include ammonium, nitrate, phosphorus, temperature, and metals (Crites and Tchobanoglous, 1998).

Although subsurface flow wetlands were referred to as vegetated submerged beds (VSB) in the first edition of Reed, et al., 1988, the term subsurface flow wetlands has been accepted and used in the industry for the last 10 years (Reed, et al., 1995).

In pond effluent the constituent of concern is often the floating algae. Both types of constructed wetlands are effective in removing suspended solids. In the free water surface wetlands the removal mechanisms include auto-flocculation, sedimentation, filtration, and die-off. For die-off to be effective, the algae must be shaded for 5 to 7 days. If open water areas are designed into a FWS wetland, the detention time cannot be more than 2 to 3 days for these areas, or else algae will regrow in the wetlands. In subsurface flow wetlands, the principal removal mechanisms are sedimentation and filtration. Because shading will be complete (as long as the flow remains subsurface), the algae and suspended solids are usually trapped within a detention time of 2 to 3 days (Crites and Lesley, 1999).

Many constructed wetlands treat effluent from facultative or oxidation ponds. A list of selected free water surface constructed wetlands treating pond effluent is presented in Table 2. Many of the wetlands were developed from existing cells of treatment ponds.

Design of Constructed Wetlands for Cle Elum, Washington

Cle Elum, Washington is located in northwest Kittitas County, approximately 25 miles northwest of Ellensburg, Washington.  The City of Cle Elum’s wastewater treatment plant serves approximately 2,300 people within the City itself and the nearby Town of South Cle Elum. Before the wetland conversion, the treatment plant consisted of three-cell lagoon followed by a chlorine contact chamber. Treated effluent is discharged to the Yakima River.

The city of Cle Elum had a history of violating their NPDES permit.  Flows and loadings to its plant were projected to grow significantly in the next twenty years, yet the treatment plant was already at capacity.  Planning efforts are underway to evaluate the relative merits of several alternative long-range plans to accommodate increased wastewater flows and loads due to population growth in the area. Conversion of the third lagoon to a wetland was chosen as an interim upgrade to handle increased treatment capacity and more stringent discharge requirements (Crites and Smith, 2000). 

The new revised permit required BOD and TSS concentrations not to exceed 30 mg/L and a minimum removal of 85 percent of the incoming BOD and TSS loads.  The treatment efficiency was the driving design parameter because of high infiltration/inflow (I/I) from the spring snow melt that creates a dilute waste stream requiring effluent BOD concentrations of 14 mg/L.  The anticipated system TSS removal is 92 percent.  The predicted system performance for BOD and TSS is shown in Tables 3 and 4.

The wetland consists of a series of three planted zones separated by two open zones.  The planted zones, totaling 68 percent of the total area, are 1.5 feet deep and are planted with hardstem bulrush, Scripus acutus, a common native wetland plant.  The open zones are 3 feet deep, which prevents growth of wetland plants.  The planted zones allow for denitrification and BOD removal.  The open zones reduce short-circuiting and allow natural aeration to increase dissolved oxygen.  Operating criteria for the treatment plant and the wetland are shown in Table 5.

Construction of the wetland occurred during the summer of 1999.  There was approximately 6 in of sludge in the bottom of the pond.  Timing was critical to allow drying of the sludge in the pond and a dormant rhizome planting in October.  The pond was drained and sludge was worked with a front loader to assist drying.  When the sludge was manageable, it was removed off-site and spread for further drying.  Later the sludge was reused as part of the top-planting medium.

To maintain the necessary hydraulic head for the outfall, the bottom of the pond was raised 2 ft.  The majority of the fill was done with an unclassified fill.  A ¾ inch minus, non-angular, fill was used for 0.5 ft below and above a HDPE liner.  The layer covering the liner was the surface of the open zones.  Another 1.5 feet of fill was added for the planted areas.  The final 0.5-foot lift of the raised zones was a mixture of 1 part fill and 1 part sludge.

Table 3.  Predicted Cle Elum System BOD Performance at Maximum Monthly Conditions

Table 4.   Predicted Cle Elum System TSS Performance at Maximum Monthly Conditions.

Dormant rhizomes were planted on an 8 in x 30 in grid 2 in deep.  Rhizomes ordered from a nursery arrived in staggered shipments.  The nursery plants averaged ¼ inch in diameter and 2 inch in length.  Due to delayed shipments, native plants from a local property where transplanted.  The transplanted rhizomes where approximately ¾ inch in diameter and were cut to contain at least one nodule for new plant growth.  The planting contractor charged on a per plant basis of approximately $1.

Start-up of Wetlands Operation

Start-up condition of the plants is critical to their survival.  The water temperature and level was monitored.  When the water temperature reached 10°C , the water level was lowered to 3 in to allow for photosynthesis of the new plant shoots.  Once the average plant height was 18 in the water was gradually be raised to the 18 in operating level over a period of three weeks.

During the summer of 2000 as the plants grew, the water level was raised. One area where plants did not grow well was subsequently replanted with transplanted local stock.. During the winter an ice cover was formed and care was taken not to raise the water level or uproot any plants. Trapping for muskrats was also required during the summer of 2000 and continues to be necessary.

Treatment Performance

Treatment performance for the Cle Elum municipal treatment plant has met and often exceeded expectations. Removal of BOD and TSS for the plant is summarized in Table 6. Comparing the predicted BOD in Table 3 of 5 mg/L soluble BOD to the measured value of 6.4 mg/L total BOD lends confidence to the design model. Similarly, the relationship between hydraulic loading rate and effluent TSS in Table 4 appears valid because the measured TSS of 3 mg/L compares well with the expected value of 4 mg/L. The open water areas also help to keep the effluent dissolved oxygen relatively high, reducing the need for post-treatment reaeration.

                        Table 6. Performance of Cle Elum Treatment System, May 2001

Acknowledgments

I would like to acknowledge our client, Larry Grimm of W&H Pacific, for making the design possible and coordinating with the City of Cle Elum. I thank Jim Leonhart, City of Cle Elum, for the operating data.

References

Crites, R.W. and D. Lesley (1999) Constructed Wetlands Remove Algae. Presented at the Annual Hawaii Water Environment Association Conference. Honolulu, Hawaii.

Crites, R.W. and J.W Smith (2000) Converting Ponds to Constructed Wetlands. Presented at the Annual California Water Environment Association Conference. Sacramento, California.

Crites, R. W. and G. Tchobanoglous  (1998)  Small and Decentralized Wastewater Management Systems, McGraw-Hill Co. New York.

Reed, S.C., R. W. Crites, and E. J. Middlebrooks (1995) Natural Systems for Waste Management and Treatment. McGraw-Hill, New York.