Large Subsurface Treatment Systems in Minnesota Brett Ballavance, P.E. MPCA, Duluth

New Wave of Wastewater Treatment

Aging Infrastructure in Minnesota  2005 WINS Study (Wastewater Infrastructure Needs Survey) –57% of MN sewer systems are 30 years old or older –14,668 miles of sewer system in the state –Statewide average age of WW treatment facilities is 22 years

Jan Report to the Legislature  Over $3.4 billion of need in the state right now for sewer systems & treatment plants  Covering over 1,000 wastewater projects  State Revolving Fund/WIF is providing $121 million this year for ~30 projects  $250-$300 million requested each year  At the same time federal funding is being reduced (cut 30% in last 2 years)

Growth of MSTSs & LSTSs in MN  Big increase in the # of LSTSs in Minnesota –~109 LSTSs in the state –Approx. 76% permitted in last 7 years  Nearly 880 domestic MSTSs in state  Agency is responding by creating new tools –Design Guidance for LSTS (3 rd version soon) –New chapter 7081 rules for MSTS

Cluster System Terms  ISTS –Individual Sewage Treatment System –Single home systems  MSTS –Medium-sized Sewage Treatment System –Cluster, less than 10,000 gpd of flow  LSTS –Large Subsurface Treatment System –Cluster, greater than 10,000 gpd of flow

Annandale- Maple Lake Court Case

Undersewered Areas in Minn. 1996

Summary of LSTSs in Minn.  109 total  Region w/ Most: Metro (36) followed by Brainerd (35)  County w/ Most: Sherburne (15) followed by Washington (12) & Scott (10)  ~2.5 million gpd total treated by LSTSs

Typical LSTS Flow Diagram  Use with the permission of North American Wetland Engineering

Secondary Treatment Tech.  Recirculating sand filters: 22  Constructed wetlands: 12  Recirculating gravel filters: 10  AdvanTex textile filters: 9  FAST units: 7  Peat filters: 4  SBRs: 2 (?)  None: 36

Starting Point – LSTS Guide

LSTS Guide Highlights  Do I need a MPCA permit?  Preliminary Site Evaluation (up front planning)  Soils Evaluation/BOD and TSS treatment  Pathogen Treatment  Nitrate Nitrogen Treatment  Areal Drainfield Sizing

LSTS Guide Highlights  Phosphorus Treatment  Hydrogeologic Evaluation  Septic Tank Sizing and Design Issues  Sewer System Design  Permit Application Submittal

Do I Need a State Permit?  30-homes requirement being eliminated ?(likely)  Old state permit threshold –10,000 gpd or –30-homes or more  New state permit threshold –Just the 10,000 gpd

Preliminary Site Evaluations  New version of guide will have more of an emphasis on first doing an up front desktop analysis.  Main goal is to identify potential problem issues & to justify a more detailed and costly evaluation of the site.  More resources listed.

Soils Evaluation  1 st & 2 nd version – PSS preferred  3 rd version – PSS required ?(likely)  Soil sizing factors –2 lists down to 1 –All for pretreated effluent –?? May change??

Pathogen Treatment  No changes to this section.  Yet we are considering requiring monitoring wells directly below the middle of the drainfield to monitor both mounding height and to take fecal samples to evaluate performance.

Nitrate Nitrogen Concerns  Some older LSTSs have high N levels in wells  Some areas have high background levels  LSTSs/MSTSs contribute to this problem  Agency developed LSTS nitrogen policy –Treat (denitrify) to 10 mg/l –Or model to show compliance at property line

Nitrogen Policy Changes  Previous policy/guide – 3 options –End-of-Pipe 10 mg/l total nitrogen –Model 10 mg/l at boundary & install gw wells –Model 5 mg/l at boundary & no wells needed  New policy/guide – 2 options –End-of-Pipe 10 mg/l total nitrogen –Model 10 mg/l at boundary & install gw wells

Tertiary Treatment - Denitrification  So far 3 technologies approved  11 total systems but 3 technologies  Fall under the heading of: –“Denitrification processes with supplemental substrate addition” –Carbon source addition, attached growth denit

Approved Denit Systems  Chemical (carbon source) addition –Acetic acid approved for 10 cases so far –“Fresh” wastewater carbon source for 11th –Methanol, “Micro C”, etc. also work  Basic concept of this type of denit –Nitrify in the secondary treatment unit –Denitrify in subsequent unit by adding a carbon source with attached growth on a substrate

Denitrification Heterotrophic Heterotrophic Bacteria Bacteria NO Organic Matter  N 2 + CO 2 + OH - + H 2 O NO Organic Matter  N 2 + CO 2 + OH - + H 2 O NO 3 - can be reduced, under anoxic conditions, to N 2 gas through heterotrophic biological denitrification as shown in the following unbalanced equation: NO 3 - can be reduced, under anoxic conditions, to N 2 gas through heterotrophic biological denitrification as shown in the following unbalanced equation:

Approved Denit Systems  NAWE Denit Unit –Nitrified in gravel filter (typical) –Denitrified in subsequent tank with carbon source addition & fixed growth on plastic media  FAST Denit Unit –MicroFAST for secondary treatment –NitriFAST – for further nitrification –ABC-N – anoxic denit. Zone  Fessel Denit Unit

Phosphorus Changes  500 feet is the new trigger for when to evaluate for phosphorus –Within 500 feet of a surface water evaluate adsorption capacity of soils for life of system  Previous policy was 300 to 2,640 based on soil texture

Linear Loading Rate  Dropping the linear loading rate section of the guide because it is thought that groundwater mounding analyses capture this concern for horizontal movement of water more appropriately.

Groundwater Mounding  All projects must evaluate potential for groundwater mounding regardless of nitrogen option chosen  Thus all LSTS projects involve some level of hydrogeo work beyond soils analysis

Hydrogeological Evaluations  Require investigations into items such as: –Hydraulic Permeability (i.e. slug tests, pumping testing, laboratory tests, estimates from grain- size distribution) –Aquifer (saturated) Thickness (deep borings, hydrogeologic atlas, nearby well boring records) –Specific Yield (literature data, laboratory tests)

Septic Tank Sizing Changes  STEP/STEG or community tanks  Third option would be a clarifier – continuous solids removal  Fourth option would be an engineered design or vendor/product-specific design –Performance-based  Monitor tank effluent for secondary treatment design compliance (lbs BOD/day)

Conclusion  LSTSs/MSTSs are likely going to continue as a part of our infrastructure  Use LSTS Guide as starting point in design  Treatment is easy – getting water in ground is often not easy (mounding)  Follow the nitrogen policy  Plan for system management and O&M  If designed, constructed, and operated well – can be a viable part of our infrastructure

Questions/Comments  For more information call or me at: –218/ –  Design Guidance for Large Subsurface Wastewater Treatment Systems (LSTS) – engineering.html engineering.htmlhttp:// engineering.html  Nitrogen Policy – 10.pdf 10.pdfhttp:// 10.pdf