Understanding Blending in Wastewater Treatment Plants

1. Where Do You Know Your Peak Flow Is?

2. OR – Why Everyone Needs to Know About Blending Presented at BACWA Wet Weather Management Workshop May 28, 2008 by Robert Cole Central Marin Sanitation Agency and Monica Oakley Oakley Water Strategies

3. Outline for Discussion • What is Blending? • What Causes Blending? • What are the Current Requirements for Blending? • Why Should I Care? • What are the Current Regulatory Developments at Regional, State, and National Level?

4. What is Blending? (during larger storm events, a portion of the flow is routed around secondary treatment units) Secondary (Biological) Treatment Primary Settling Tank To Water Body Disinfection and Dechlorination (if necessary) Plant Flow from Headworks Basic Diagram for Blending

5. What Causes Blending? • In some plants, primary treatment capacity is larger than secondary capacity. • High wet weather flows arrive at plant from sanitary sewers. • Wet weather flows in sanitary sewers come from many sources, including: • Leaks in pipe joints and connections • Broken or cracked sewer pipes • Leaky manhole walls • Illegally connected roof drains, foundation drains, catch basins, area drains, and manhole covers

6. Current Requirements for Bay Area Treatment Plants That Blend • Plants must prepare a No Feasible Alternatives Analysis (NFAA), and (preferably) submit with ROWD for permit renewal. • If plant does not submit NFAA with ROWD, Regional Water Board staff will issue 13267 letter during permit renewal, with short lead time for preparation. • Special NPDES permit language will require continued activities to reduce blending during permit term (5 years). • Plants must prepare another NFAA every five years to show reduction in blending.

7. NFAA Requirements (summarized) • Document design capacity for all treatment units, maximum flows, and feasibility and cost of increasing treatment capacity. • Estimate frequency, duration, and volume of blending events; evaluate alternatives and costs to reduce. • Estimate potential for future blending events based on collection system changes. • Consider expansion of storage within the collection system or on-site and related costs. (continued next slide)

8. NFAA Requirements (summarized) - continued • Assess other ways to reduce peak wet weather flow volumes, such as limiting collection system expansion. • Evaluate technologies that could be used for more treatment to peak flows and associated costs. • Evaluate whether I/I reduction is being maximized throughout the entire collection system, and include options for expanding legal authorities to reduce I/I from satellite collection systems. • Evaluate peak flow reductions obtainable through implementation sewer system management program.

9. Why Should I Care About Blending? • Reducing wet weather flows in sewers is an important goal of regulatory agencies to control blending. • If plants don’t blend, flows may back up into sewers and cause overflows. • Wet weather requirements are being applied directly to collection system agencies by: • USEPA enforcement orders • Third party lawsuits • Other enforcement expected in future • Collection system requirements under these mechanisms are similar to some blending requirements.

10. Current Blending Regulatory Developments • Regional: NFAA’s required, as well as special requirements in NPDES permits • State: No direct activities for blending at this time • National: The current administration recently pronounced, “no more regulations if the proposed version is not issued by June 1…” so new national requirements not likely within a year

11. Each Agency Needs To Do Their Part While wastewater treatment plants are being required to reduce blending, additional attention is also being given to the source of wet weather flows – the collection systems.