There are four basic stages to cleaning wastewater at MMSD's reclamation facilities.
- In the first stage, wastewater enters preliminary treatment where screens and grates remove large objects, sand, gravel and other grit.
- In step two, primary treatment, the wastewater enters large settling tanks where we remove grease and oil after it floats to the top. Heavier material sinks to the bottom of the tank and is removed.
- In secondary treatment, microscopic organisms, or "bugs", breakdown the majority of organic material that remains in the wastewater. Keeping organic material out of rivers and lakes is important because it can consume large amounts of oxygen that fish and plants need to live.
- Finally, the water goes through disinfection. It's here that chemicals kill disease-causing organisms. The chemicals are then removed before the water is discharged to Lake Michigan.
What is Blending?
MMSD uses blending as a safety valve” to prevent basement backups, raw sewage overflows and damage to the Jones Island Water Reclamation Facility.
Sometimes referred to as a diversion, blending helps protect the secondary treatment process that uses microscopic bugs to break down pollution in wastewater.
Bugs are the workhorses of water reclamation and they don't like it if too much water is sent through their home.
Too much flow can physically wash the bugs out of the facility, polluting Lake Michigan and possibly leading to the loss of secondary treatment for weeks.
The Blending Process
- All rainwater and sewage goes through screening and primary Clarification.
- Blending becomes necessary when too much extra water threatens to cause basement backups, sewage overflows or damage to secondary treatment.
- When the decision is made to blend, 20% or less of the total flow is diverted around secondary treatment and blended back together with flow that did go through secondary.
- All flow (100%) then goes through disinfection and must meet federal and state standards for full treatment.
In 2008, MMSD will conduct a full scale pilot study of a disinfection process modification at the South Shore Water Reclamation Facility. The purpose of the pilot study is to evaluate the addition of a small amount of ammonia to the secondary effluent to improve the efficiency of the wastewater disinfection process. The pilot study at South Shore will employ the same methods (addition of ammonium sulfate to secondary effluent) used in studies at the Jones Island Water Reclamation Facility in 2006 and 2007. In addition, MMSD will operate the Jones Island process change in 2008. The intended schedule for the South Shore pilot study and the Jones Island operation change is July 1 to October 15, 2008. The Wisconsin Department of Natural Resources has given their approval to the studies and operational changes.
The results of the full scale studies conducted at the Jones Island Reclamation Facility in 2006 and 2007 indicated a high level of disinfection efficiency and no adverse impact to effluent quality. Disinfection process efficiency is monitored in terms of the Fecal Coliform result (a WPDES permit daily requirement/limitation), production of disinfection byproducts (e.g. chloroform), and the dosage and cost of the disinfection and de-chlorination chemicals (sodium hypochlorite (chlorine bleach) and sodium bisulfite, respectively). In the summer months, where the nitrification reactions in the secondary treatment process typically reduce effluent ammonia concentrations to less that than 0.5 mg/L, the disinfection chemical dosages and costs and the plant effluent chloroform concentrations have been higher than at any other time of the year; the inefficiencies have not caused a Fecal Coliform permit violation.
Before the process can become permanent, MMSD will submit plans and specifications to the DNR for review and approval.