Construction & siting issues  
  1. 1. Runoff enters a fore bay and then passes into a series of compost treatment cells. Each cell contains a one foot depth of compost, followed by a filter fabric, a six inch layer of small diameter rock, and two inches of pea gravel.
  2. Runoff filters through the compost and is then collected by a perforated pipe and directed toward the outlet. The slope from the inlet to the outlet of the hundred foot long filter bed is two percent and requires about three feet of head.
  3. Like most storm water filtering systems, the filter bed and subsoils are separated by an impermeable polyliner.
  
   

* This filter system served a 74-acre mixed residential watershed and was sized to provide 200 square feet of surface area per cfs of incoming flow. The target for runoff treatment is to capture one-third of the two year design flow. This translates to about .10 watershed-inches of storage, assuming a 2.25 gpm/ft squared rate for the first 30 minutes of runoff.

  
   
 
Critical Factors:  
The key to good performance is proper selection of compost. Here are some ideal characteristics. Note that for the Sand Point Wetlands project, the surrounding neighborhoods could supply a good quantity of fallen leaves for compost.  
   
  • Mature organic litter that no longer rapidly degrades
  • Hemic - probably blood meal or blood waste
  • Low contaminant levels
  • High permeability
  • Locally obtainable at a reasonable cost
  
 
   
 
   
Drawbacks/Need for Improvement:  
Total dissolved solids, however, increase after passing through a leaf compost filter, which appears to reflect the exchange and/or leaching of cations (positively charged ions of calcium, nitrogen, or ammonia) within the composts. While these particulate nutrients are trapped within the compost, the system exports soluble phosphorous and nitrates.
Subsequent monitoring in 1992 has confirmed that these removal rates can be equaled or exceeded.
 
   
Window of Opportunity for Best Function:  
In general, this system was most effective during the first flush of runoff and smaller storms, with removal rates declining as storm size increased. Better removal rates can probably be attained by increasing either the surface area or storage volume of the compost system.

 
   
Maintenance:  
The compost system requires annual or biennial removal and disposal of the compost layer, followed by replacement of fresh compost. It can cost up to 1,000 dollars. Tests indicate that the compost can be safely land filled.
  
   
Maintenance Problems:  
The key problem has been sediment deposition over the surface of the compost that reduces the permeability rate. Perhaps the use of larger fore bays, lower design permeability rates, or regular raking/discing of the filter bed surface could relieve the problem. W&H Pacific are refining the design.