Introduction To Wastewater Treatment Options For Small Communities

TechnologyFeaturesAdvantagesLimitationsManagement and MaintenanceSeptic TankEffluentPump(STEP) Each home has its own septictank, or several homes may beconnected to large septic tank. Effluent flows by pressure outof septic tank into mainlines. Typically uses 2 – 4” pipe. Uneven terrain and uphill collection possible. Relatively easy to expand system. Solids are retained in septic tank so reducedcleaning of mainlines, treatment facility isneeded. Smaller pipe size, shallow depth of burial. Reduced infiltration/inflow. No access ports in street. Allows for easier diagnosis of problem users. Leaking septic tanks may causeinfiltration issues. Septic tank on property undesirable tosome property owners. Service by a Minnesota licensedISTS professional at least every3 years or as required by LGU. Effluent filter cleaning permanufacturer recommendations(typically annual). Pump replacement as required.VacuumSewer Valve pits located every 2-4homes. A small diameter pipecarries the effluent to a vacuumpump station. Typically uses 2 – 4” pipe. No power is required at valve pits.Leaks are minimal and easily detected.Shallow installation.No manholes.Useful for high water tables or rocky terrain. Not appropriate for areas with greaterthen 30 feet of elevation difference. Economically feasible for 80connections or more. Must be within 2 mi. of vacuum pump. Servicing, maintenance on aregular basis by a licensedoperator.Regionalization or Combining with Another Community:may include expanding an existing system in one or both communities, orestablishing a new joint system using any of the technologies listed in this document. Connect communities toexisting or building newwastewater treatment facilities. May utilize a treatment systemincluded in this document. May be higher service, connection Community ties into an existing facility – cancharges. Requires trained, licensed Large May require joint powers or annexationbe efficient for management issues.System Operator. May stimulate community growth.agreement. May stimulate community growth.Secondary Treatment: individual systems or decentralized systemsMost come between the septic tank and the final treatment/dispersal method. Most commonly, dispersal is via a soil—based treatment method.Who designs: Qualified Minnesota Professional Engineer or qualified MPCA licensed Designer, depending on system chosen, size and other factors.Who regulates: Usually LGU if less than or equal to 10,000 gal. per day flow. MPCA if over 10,000 gal. per day.AerobicTreatmentUnit (ATU) Aerobic treatment process is 10-20 timesfaster than anaerobic; more efficient. Uses aerobic microorganisms May be able to retrofit an existing system.to provide secondary treatment, Less space needs for pre-treatment than otherremoval of biodegradablesecondary treatment options.organics and suspended solids. Available for ISTS, cluster or municipal use. All types need an air supply to Option: can combine with membranes forcreate aerobic environment.advanced bacteria removal. Aeration chamber may contain Provide reliable secondary treatment.attached-growth (organisms Reduces the biomat in soil treatment area.attach to media) and/or May allow for higher loading rates in thesuspended-growthdispersal area.microorganisms (microbes May allow for wider range of soil types in thefloat freely and are settled).soil dispersal area. Media can be easily cleaned if plugged.Page 4 of 14 Prefers a steady supply of food tomaintain stable microbial population. ATU’s are sensitive to drain cleaners,antibiotics, antibacterial products. Limited nitrogen and phosphorusremoval with standard design. Additional components are needed forsignificant nitrogen or phosphorusremoval. Higher energy costs due to air source. Location of air source: consider noiseissues, temperatures, snow impacts. Attached growth systems can plug ifoverloaded with solids (may be due tolack of maintenance or issues withuse). Manufacturer of ATU requiresspecific management ofcomponents. Generally, Minnesota licensedISTS professional typicallyprovides bi-annual or quarterlymonitoring for individualsystems. Cluster systems –generally more often servicerequirements. Generally, higher maintenancecosts and more intensivemanagement needed than someother pre-treatment choices. Must follow 7080 or localgovernment requirements.

TechnologyConstructedWetlandFeatures Lined, constructed area usesanaerobic bacteria to treatwastewater. Cell is filled with 20-24” solidmedia – usually gravel. Wetland vegetation is planted. Used to recreate the treatmentprocesses that occur in naturalwetlands.AdvantagesLimitationsManagement and Maintenance Microorganisms, media and plants providesecondary treatment. Appropriate option for communities orindividuals where adequate affordably pricedland is available. Wetlands have esthetic appeal. Provide easier operation than some types. Provide reliable treatment. Reduces the biomat in soil treatment area. May allow for higher loading rates in thedispersal area. May allow for wider range of soil types in thesoil dispersal area. Larger land requirements than othermedia filter systems. More susceptible to freezing – must bedesigned for temperature control. Vegetation needs to be managedincluding plant diversity, weedmanagement; re-planting if necessary. Vegetation needs to be harvested ifnutrient removal is a goal. Full treatment capability not reacheduntil vegetation is established. Media may plug if overloaded withsolids due to issues with use or lack ofmaintenance. May require higher frequency ofmaintenance than other options. Generally, Minnesota licensedISTS professional typicallyprovides annual monitoring forindividual systems. Clustersystems – generally more oftenservice requirements. Must follow 7080 or localgovernment requirements. Designed and operated for advanced nitrogen Generally, Minnesota licensedremoval. Use a sand, gravel, peat orISTS professional typically Some systems send effluent back to septicsynthetic textile filter with aprovides annual monitoring fortank versus recirculating tank for increased Requires more pumps and controls thanrecirculation tank.individual systems. Clusterdenitrification.single pass media filters, increasingsystems – generally more oftenRecirculating Mixes a portion of filtered Smaller land area requirement.management requirements.effluent with incoming effluentMedia Filterservice requirements. Provide reliable secondary treatment. Media may plug if overloaded withfrom the septic tank.(RMF) Manufacturer of pre-treatment Reduces the biomat in soil treatment area.solids due to issues with use or lack of Effluent passes through theunit requires specific component May allow for higher loading rates in themaintenance.filter several times before goingmanagement.dispersal area.to pressurized soil treatment. Must follow 7080 or local May allow for wider range of soil types in thegovernment requirements.soil dispersal area.Single PassMedia Filter(SPMF) Use natural, mineral orsynthetic media such as sand,gravel or peat; in lined cells, orprefabricated containers. Wastewater is treated as itmoves over the media surfaces.Harmful organisms and organicmaterial are removed from thewastewater in the media filter. Typically have effluent filter,pumps, control panel. Ease of operation and easier start up. Effective for removing organic matter andpathogens. Provide reliable secondary treatment. Reduces the biomat in soil treatment area. May allow for higher loading rates in thedispersal area. May allow for wider range of soil types in thesoil dispersal area.Page 5 of 14 Peat deteriorates over time – must bereplaced. Requires dosing pumps, timers andcontrols. Can require more area than ATU orRMF options. Media may plug if overloaded withsolids due to issues with use or lack ofmaintenance. Generally, Minnesota licensedISTS professional typicallyprovides annual monitoring forindividual systems. Clustersystems – generally more oftenservice requirements. Manufacturer of pre-treatmentunit requires specificmanagement of components. Must follow 7080 or localgovernment requirements.

TechnologyFeaturesAdvantagesLimitationsManagement and MaintenanceMechanical (Municipal) Treatment Plants, Large Aerobic Treatment Units and Pond SystemsPreliminary Treatment – Physical separation (removal of materials such as debris, rocks, etc.). Impacted by type, number of septic tanks, if any.Primary Treatment– Separate solids from effluent via a variety of methods such as sedimentation tanks that use filtration and settling procedures.Secondary Treatment– An aerobic, biological process designed to remove nutrients, especially ammonia. Includes the methods listed below, andthe pre-treatment options listed above (usually called “pre-treatment” when discussing onsite treatment methods; secondary treatment in municipal).Tertiary Treatment– Additional treatment to remove a specific nutrient (Phosphorus or nitrogen), reduce Biological Oxygen Demand (BOD), Total Suspended Solids (TSS).BOD and TSS should be 30 mg/l. and/or pathogens.Final dispersal– Soil based or surface water discharge. Requires NPDES/SDS permit from the MPCA. Most systems create sludge, which must be dispersed.Who designs: Qualified Minnesota licensed Professional Engineer.Who regulates: MPCA for all surface water discharges. Requires MPCA professional operator license, based on technology chosen.Secondary or Pre-Treatment Wastewater Treatment Options for large clusters or municipal treatment. Typically, thesemethods utilize surface-water discharge methods, some use soil-based methods. See previous section for additional pre-treatment options.ActivatedSludge Functions similar to aerobic Requires the smallest space of the municipaltreatment unit – is much larger.options. Often called “Package plant.” Mechanical aeration providesoxygen for treatment. Discharge may be continuousAerated Pond or spring/fall, depending onreceiving waters and permits. Typically use surface waterdischarge.RotatingBiologicalContactor An aerobic treatment unit. Media disks pick upwastewater as they rotatethrough the wastewater. Organisms use organic matterin wastewater for food. Used by many smallcommunities. Consist of largelagoon cells about 9’ deep foroxidation of organics andStabilizationsettling, then a dischargePondmethod – typically spring/fall(Lagoon)discharge to surface waters. Ponds use microbes existingnaturally in water for treatment. High sampling and management costs. More mechanical parts. May be higher noise, odor concerns. Requires a disinfection process forpathogen removal. Usually not suitable for high flowfluctuations. Smaller footprint (since it is deeper) – smallerland requirement. Has more mechanical parts so highermanagement needs. Ammonia removal is more consistent than Provides medium performance level.other choices. Usually are fewer odor issues than other Higher energy costs.choices. Medium management costs. Medium – high performance levels. Higher energy costs – a mechanicalsolution. Lower capitol installation costs than someother choices. Lower operational requirements than otherlarge systems. Reduced sludge dispersal issues. Uses natural disinfection process. Non-mechanical solution. Solids are stored in the ponds until disposal. Larger land requirements. May have higher algae concentrations– may impact surface water discharge. May have seasonal odor control issues. May be aesthetic issues. Lower quality discharge. Low performance system. Higher risk for public safety issues.Page 6 of 14 Requires a trained, licensedLarge System Operator. Most need monitoring on afrequent basis.

TechnologyFeaturesTricklingMedia Filter Wastewater is distributed overaerated media; trickles down. Have a settling basin (clarifier)to capture solids.Advantages Low – medium management costs. Medium performance levels. Same principle as media filters.LimitationsManagement and Maintenance More likely to have odor problems Need a building for temperature, odorcontrol, mechanical components.Final Treatment and DispersalWho designs: Qualified Minnesota Professional Engineer or qualified, MPCA licensed Designer, depending on system chosen, size and other factors.Who regulates: Usually LGU if less than or equal to 10,000 gal. per day flow. MPCA if over 10,000 gal. per day.Subsurface Land DischargeAt-GradeBed Use pressure dosing into a rock Higher level of maintenance than trenchesbed.due to pumps. Community systems may use a Common system with much experience,series of at-grades.expertise available. Large land area requirement. Require pumps, controls. Has potential for groundwatermounding. Requires soil conditions with adequateseparation (greater than 3 feet). Backup site needed for replacement. Servicing by a Minnesotalicensed ISTS professional atleast every 3 years or as requiredby LGU for individual systems,more frequently for clustersystems. Check caps, look for surfacingeffluent, manage water use andwatch for leaks in householdplumbing. It is recommended that cluster orcommunity systems have a legalentity for management. This isrequired if the communityreceives public funding.Mound Creates separation by addingclean sand above ground. Community systems may use aseries of mounds. More effective at removing nitrogen thanother choices without addition ofdenitrification processes. Higher level of maintenance than trenchesdue to pumps. Common; experience and expertise available. Large land area requirement. Requires soil conditions with adequateseparation (greater than 1 foot). Backup site needed for replacement. Require pumps and controls. Potential for ground water mounding.Trench andBed Used when there is sufficientunsaturated and permeable soilbeneath the drain field, andadequate space for the system. Community systems may use aseries of trenches. May use rock, chambers orgravelless pipe in the trenches. Least expensive to install and maintain – canuse gravity flow (some systems – especiallylarger community systems are pressurized). Systems are flexible for sizing, placement. Effective at removing pathogens, phosphorusand ammonia. Lower installation and maintenance costs. Large land area requirement. Requires soil conditions with adequateseparation (greater than 5 feet). Backup site needed for replacement. Require pumps, controls if pressurized. Potential for ground water mounding.Shallow, uses soils more likely to be aerobic.Maximizes use of evapo-transpiration.Timed dosing prevents over-saturation.More uptake by plants since is shallow.Can be used in more slowly permeable soilssuch as in wooded areas, allowingcommunities to preserve natural areas. Encourages water re-use. More nutrient reduction than some methods. Monitor regularly for plugging Effluent must be well filtered – smalland freezing issuesholes in piping plug easily. Check caps, look for surfacing Shallow, so can be freezing issues ineffluent, manage water use andthe pipes.watch for leaks in household Rodents can cause problems.plumbing. Requires proper installation to prevent Inspection and servicing by afreezing.Minnesota licensed ISTS Maintenance is critical to success.professional. Typically, annually Additional management needs if manyor more frequently for clusterhomes are unoccupied in winter – moresystems.susceptible to freezing.DripDistribution Generally follows secondarytreatment systems. Always has an effluent filter inthe design. Disperses wastewater over alarge subsurface area; adding tosoil slowly and shallowly. Page 7 of 14

TechnologyFeaturesAdvantagesLimitationsManagement and MaintenanceSurface Water DischargeSurfaceWaterDischarge Discharges into a local surfacewater body - usually a river,ditch or stream. Requires NPDES/SDS Permitfrom MPCA (see glossary). Requires acceptable receiving watersclose to the treatment area. Receiving waters can accept large flows. Higher management requirements. Do not need soil on site to accept wastewater. Additional disinfection may requirechlorination or UV methods. MPCA Licensed Operator. Regular testing by licensedoperator to ensure permitrequirements are being met; highmaintenance requirements.Surface Discharge to Land Discharges to a land surface via Maximizes use of evapo-transpiration.SprayDistributionirrigation

Introduction to Wastewater Treatment Options for Small Communities All wastewater treatment systems begin with the basic premise of wastewater collection followed by treatment and dispersal. There are several collection, pre-treatment, treatment, final dispersal or water recycling options for communities as noted in the chart on page 9.