Septic system design and installation field guide

What a septic system is, and where the work really lives

A septic system handles wastewater on the property when there is no public sewer to take it. Every drain in the house, the toilets, the sinks, the showers, the washing machine, runs to one buried pipe that leaves the building and feeds a tank in the yard. The tank holds the waste, settles out the solids, and lets a clarified liquid called effluent flow on to a buried bed of soil called the drainfield. That is the whole system: a tank that separates, and a field of soil that treats.

The part that surprises people, including some who install these for a living, is where the actual treatment happens. The tank does not clean the water. It settles and stores. The cleaning, the part that turns sewage into something safe to release back to the ground, happens in the soil under the drainfield. That single fact drives every real decision on the job, and it is why a soil evaluation comes before a single line is drawn.

Two siblings sit next to this guide. Where a public sewer is available, the building ties into it through a gravity lateral instead of a septic system, and that connection has its own rules for slope, bedding, and the tap to the main, covered in the building sewer lateral guide. Where the drain leaves the building below the level of the field or the sewer, the waste has to be pumped, and the sewage lift station guide covers that. Septic is the case with no sewer at all, where the soil on the lot is the only treatment plant you get.

The soil is the treatment, not the tank

This is the one idea that separates someone who designs septic systems from someone who just digs holes: the soil does the treatment. The septic tank is a settling chamber. It drops the heavy solids to the bottom and floats the grease to the top, and it sends a cloudy but solids-free liquid out the other end. That liquid is still full of pathogens, nitrogen, and organic load. It is not treated. It is only separated.

The treatment happens after the effluent leaves the tank and soaks into the soil under the drainfield. As it percolates down through unsaturated soil, a thin biological layer called the biomat forms at the soil interface, and the bacteria, the soil chemistry, and the filtering action of the soil grains break down the organics, consume the pathogens, and pull nutrients before the water reaches groundwater. The soil is the filter and the reactor at once.

Everything else follows from this. If the soil cannot accept and treat the effluent, no tank, no pump, and no gadget fixes it. That is why a bad site cannot be bought back with a bigger tank, why driving over the field destroys the system, and why the soil evaluation is the first thing that happens, not the last. Get the soil right and the rest is plumbing. Get the soil wrong and you have built a holding tank that overflows into the yard.

How does a septic system work?

A conventional septic system runs in two stages, and the waste only moves one direction. Stage one is the tank. Wastewater enters, slows down, and separates: solids sink into a sludge layer on the bottom, grease and lighter material float into a scum layer on top, and the clarified effluent sits in the middle. Anaerobic bacteria in the tank start breaking down the sludge, which is why the tank never fills completely with solids, though it does fill slowly enough that it has to be pumped.

Stage two is dispersal and treatment. The middle layer of effluent flows out the tank's outlet, usually to a distribution box that splits it evenly among the drainfield trenches, then into perforated pipe or chambers laid in soil. From there it seeps into the ground, and the soil finishes the treatment as the water moves down toward the water table. In a conventional gravity system the whole path runs downhill on slope alone, with no pump.

The sequence is the design. Settle in the tank so solids never reach the field, then spread evenly so no one part of the field is overloaded, then treat in the soil. Break any link and the system fails at the soil, because that is where the consequences land. Solids that escape the tank clog the field. Effluent dumped to one trench drowns that trench. The field is where the bill comes due.

What is a perc test and soil evaluation, and why does it come first?

A soil evaluation tells you whether the lot can treat wastewater and, if so, how fast. It is the first thing done on any septic project, before the design, before the permit, before anyone talks about tank size, because it sets the feasibility and every number that follows. Two pieces make it up. The percolation test, the perc test, measures how fast water drains out of a hole in the native soil, reported as minutes per inch. The soil profile, dug as a pit or bored, reads the soil texture, the structure, the depth to bedrock or other limiting layer, and the seasonal high water table, usually identified by mottling, the gray and orange streaks that show where water sits part of the year.

The perc rate sets the loading rate, how many gallons per square foot per day that soil can accept. Fast sandy soil percs quickly and needs a smaller field, though soil too fast can pass water before it is treated. Tight clay percs slowly and needs a large field or an alternative system. The profile sets whether you can build a conventional field at all, because the code requires a minimum depth of unsaturated soil, commonly on the order of 2 to 4 ft, between the bottom of the field and the limiting layer so the effluent has soil to travel through before it hits rock or water.

This is the part to hedge hard and never fake. The procedure, the required separation distances, who is allowed to perform the evaluation, and how the results convert to a design are all set by the local health code and carried out by a licensed soil evaluator or engineer, and the timing matters because a perc run in dry summer soil can read far better than the same ground in wet spring. Do not design off a soil eval you did not get done properly, and do not let anyone start a system on a lot that has not been evaluated and approved by the AHJ.

When a site cannot take a conventional system

Some lots will not support a conventional gravity septic system, and the soil evaluation is what tells you before the money is spent. The common disqualifiers are a high seasonal water table, shallow bedrock, and tight clay soils that perc too slowly to accept the flow. A small lot can fail on geometry alone, because there is no room to fit the field plus the reserve area at the required setbacks from the well, the house, and the property lines.

A failed perc or a shallow limiting layer does not automatically mean no build. It usually means the conventional field is off the table and an alternative system is the path, an aerobic unit, a mound, drip dispersal, or pressure dosing, each of which exists to make a poor site work at higher cost and more maintenance. Sometimes the answer is genuinely no, the lot cannot treat wastewater safely and no system the code will approve fits, and a buyer needs to hear that before closing rather than after.

The soil decides, and the health code decides what the soil allows. Whether a marginal site can be made to work with an engineered design is a call for the soil evaluator, the design engineer, and the AHJ, not something to eyeball from the road. The honest service to a client is the early evaluation, because a lot that will not perc is worth knowing about while it is still someone else's problem to price.

The septic tank: settle the solids, float the grease

The septic tank is a buried watertight container, and its whole job is to separate. Wastewater comes in, the flow slows almost to a stop, and physics sorts it. Heavy solids settle to the bottom as sludge. Fats, oils, and grease float to the top as scum. The clarified effluent in the middle is what flows out to the field. The tank also runs an anaerobic digestion that slowly reduces the sludge, but it never reduces it to nothing, which is why pumping is not optional.

Retention time is the design quantity. The tank has to be large enough that water sits long enough to separate before it reaches the outlet, which is why codes size the tank well above the daily flow, commonly enough for two to three days of detention. A common minimum for a three-bedroom house is 1,000 gallons, with larger tanks for more bedrooms, but the required size is set by the health code, not by this rule of thumb.

Concrete and plastic are the usual materials. Concrete tanks are heavy, durable, and the long-standing default, though they can corrode over decades where sewer gas creates acid above the waterline. Plastic and fiberglass tanks are lighter and do not corrode, but they have to be installed and backfilled carefully so they do not float, shift, or collapse, and some jurisdictions restrict them. Two compartments, or two tanks in series, settle better than one and protect the field. The tank material and configuration the AHJ accepts is a local question.

Baffles and the effluent filter: keep solids out of the field

Inside the tank, the inlet and outlet baffles do quiet, important work. The inlet baffle, usually a sanitary tee, directs incoming flow down into the tank instead of letting it shoot across the surface and stir up the scum. The outlet baffle, another tee, draws effluent from the clear middle layer rather than the scum on top or the sludge on the bottom. Lose a baffle, which happens when an old concrete one corrodes off, and the tank stops protecting the field.

The effluent filter is the single cheapest piece of insurance on the whole system, and the one most often missing on older installs. It is a screened cartridge that fits in the outlet tee and catches the fine suspended solids that would otherwise carry out to the drainfield. Solids in the field are what clog the soil and kill it, so a filter that costs a small fraction of a new drainfield is keeping the expensive part alive.

The catch is that a filter only works if it gets cleaned. As it does its job it clogs, and a fully clogged filter slows the tank's outflow, which shows up as slow drains in the house, a nuisance that beats a ruined field by a wide margin. Plan to pull and rinse the filter on a schedule, commonly every 6 to 12 months or at each pumping. A field without a filter is a field on borrowed time. A filter nobody cleans backs up the house.

How is a septic system sized?

A septic system is sized to the home's design flow, and flow is estimated from the number of bedrooms, not the number of people, because bedrooms are a fixed proxy that survives a change of owner. The health code assigns a design flow per bedroom, commonly in the range of 100 to 150 gallons per day per bedroom, and the whole system is built to that number. A three-bedroom house at 150 gallons per bedroom is a 450 gallon-per-day design, and both the tank and the field are sized from it.

The tank gets sized from flow and retention time. The field gets sized from flow and the soil's loading rate, which is the gallons per square foot per day the evaluated soil can accept. The math is simple once you have both: field area equals design flow divided by the loading rate. Fast soil with a high loading rate needs less area. Slow soil needs more. The perc result is what makes that division come out one way or the other, which is the whole reason the soil eval comes first.

Treat the numbers here as the shape of the calculation, not as the values to design from. The design flow per bedroom, the minimum tank size, the loading rates for each soil class, and the required field area all come from the local health code's tables and the soil evaluator's findings, and they vary by state and county. Size to the code's flow and the evaluated loading rate, and have the design approved before anything goes in the ground.

The distribution box and even spreading

Effluent leaving the tank usually passes through a distribution box, the D-box, before it reaches the field. The D-box is a small buried chamber with one inlet from the tank and several outlets, one to each drainfield trench, and its job is to split the flow evenly so every trench takes its share. Even distribution is what lets the whole field do its work. Send most of the flow to one trench and you overload that trench while the others sit dry, then the overloaded one fails first.

The D-box has to be level, and it has to stay level. A box that settles or heaves after backfill tips the flow to the low outlets, and the field loads unevenly from day one. Some installers set flow-leveling devices or speed levelers in the outlets to dial in the split. On sloped sites, drop boxes or a serial distribution that fills one trench before spilling to the next are alternatives the design may call for.

When a field starts failing at one end, the D-box is the first thing to check. A box knocked off level, a root mass in an outlet, or solids that escaped a filterless tank and settled in the box will all skew the split. It is a cheap component that controls an expensive one, so it is worth getting level on install and worth opening when the field misbehaves.

The drainfield: where the treatment happens

The drainfield, also called the leach field or soil absorption field, is the treatment plant. It is a set of shallow trenches in the native soil, each holding a distribution pipe over a bed of washed gravel, or a run of plastic chambers, with the trench bottom sitting at the depth the soil evaluation called for. Effluent trickles out along the trench, soaks down through the soil, and is treated by the biomat and the soil as it goes. The field is not a drain. It is the part doing the work this whole guide is about.

The field is sized by area, and the area comes from the flow and the soil's loading rate. The depth and the separation are set by the limiting layer, because the code requires a minimum thickness of unsaturated soil, commonly a few feet, between the bottom of the trench and bedrock or the seasonal water table. That separation is the soil the effluent travels through before it reaches groundwater, and shorting it is shorting the treatment, which is exactly what the inspector is checking when the field elevations are shot before backfill.

Two things keep a field alive: solids stay out and the soil stays open. Solids come from a failing tank or a missing filter, and they plug the soil at the trench. The soil stays open only if nobody compacts it, paves it, builds on it, or lets roots invade it. A drainfield is fragile in a way a pipe is not, because its treatment depends on soil structure that a single pass of a loaded truck can destroy. Protect it like the expensive, unrepairable thing it is.

Gravel-and-pipe trenches versus chamber systems

There are two common ways to build the field. The traditional method is a trench with 6 to 12 in of washed gravel, a perforated distribution pipe laid on the gravel, and fabric over the top to keep soil out, all in a trench commonly 2 to 3 ft deep and 1 to 3 ft wide. The gravel stores and spreads the effluent and keeps the trench open. The modern alternative is chambers, arched open-bottom plastic units that sit directly on the trench soil with no gravel, creating an open cavity over the infiltrative surface.

Chambers have taken over a lot of new work for practical reasons. There is no gravel to haul and place, the install is faster, and the empty arch stores a surge of flow. Some codes credit chamber systems with a reduced field size compared with gravel because of the open infiltrative surface, though the real treatment still depends on the area of soil in contact with the effluent. Less soil area means less treatment, whatever the storage volume, so the area reduction the code allows is the number that matters, not the marketing.

Whichever way the field is built, the infiltrative surface, the soil at the bottom and sides of the trench, is what treats. Smear that surface with a loaded bucket in wet conditions and you seal it before the system is even covered. The choice between gravel and chambers is a cost and logistics call within what the AHJ approves. The thing that does not change is protecting the soil interface during the dig.

The reserve area, set aside for the replacement field

Most codes require a reserve area, also called a repair or replacement area, set aside next to the primary field. It is suitable, evaluated soil, held empty so that when the original field reaches the end of its life, there is somewhere approved to build the replacement without re-evaluating the whole lot or finding the property has no room left. A drainfield is not forever, so the reserve is the plan for the day it isn't.

The reserve has to be protected exactly like the field itself. That means no driving on it, no building over it, no patio or pool or shed, and no parking, because compacting or covering the reserve quietly destroys the one place the future system was supposed to go. It is common to find a reserve area that got paved or built over years after the install, which turns a routine field replacement into a problem with no easy answer.

How much reserve, where it sits, and whether it is required at all are set by the local health code, and on a tight lot the reserve can be the constraint that decides whether the project fits. Lay it out on the site plan, mark it, and tell the homeowner what it is for, because the person who buys the house in ten years will not know unless the record says so.

Conventional versus alternative systems

A conventional system is a septic tank feeding a gravity drainfield, and on a good site it is the cheapest, simplest, and most durable choice, with almost nothing to maintain but the tank pumping and the filter. When the site is good, conventional wins on every count. The alternatives exist because not every site is good.

Alternative systems are engineered solutions for sites a conventional field cannot handle: high water table, shallow bedrock, tight or too-fast soil, small lots, or proximity to a sensitive water body. They cost more to build and far more to maintain, and most of them add a pump, a control panel, and a service contract. The trade is that they make a buildable lot out of one that would otherwise fail, by treating the effluent harder before dispersal, by building the soil up, or by dosing it out more evenly. The right choice is site-specific and is the soil evaluator's and engineer's call, approved by the AHJ.

The table below is the rough map of which system answers which site problem. It is orientation, not a selection rule, because the design that a given lot actually requires depends on the soil evaluation and the code, and an engineer signs off on the alternative.

The aerobic treatment unit (ATU)

An aerobic treatment unit is a small sewage plant in the yard. Where a conventional tank runs anaerobic, an ATU pumps air into the treatment tank so aerobic bacteria, which work faster and treat further, break down the waste before it ever reaches the soil. The effluent leaving an ATU is cleaner than effluent from a plain septic tank, which is why an ATU can work on a site with poor soil, a high water table, or a field too small for conventional treatment, sometimes with a reduced dispersal area.

The cost of that performance is maintenance. An ATU has an air blower or aerator running continuously, controls, and often a disinfection step, and all of it needs power and service. Most jurisdictions require a maintenance contract on an ATU and periodic sampling, because an aerobic unit that loses its blower quietly drops back to worse-than-septic treatment without telling anyone. The mechanical parts are what fail, and they fail silently.

An ATU is the right answer when the site forces it, not a default upgrade. It treats harder, but it adds a machine that has to be powered, serviced, and inspected for the life of the system. Whether an ATU is required or allowed, and what level of treatment and maintenance the code attaches to it, is a question for the engineer and the AHJ.

Mound systems and pressure dosing

A mound system builds the drainfield up above the natural grade in a constructed bed of imported sand. It is the answer when there is not enough good soil below grade, because the water table is high or bedrock is shallow, so you create the unsaturated soil column on top of the ground instead of in it. Effluent is pumped from a dose chamber up into the mound, where it filters down through the sand and then into the native soil. A mound is visible, it takes space, and it depends on the pump, but it makes a site work that has no buildable depth.

Pressure dosing is a technique more than a system type, and it shows up on mounds and on flat or large conventional fields. Instead of effluent trickling out by gravity to whichever trench is closest, a pump pushes a measured dose through a network of small-orifice pipe so the whole field gets an even, periodic load. Even dosing keeps any one area from being constantly wet, gives the soil time to drain and re-aerate between doses, and extends the life of the field. The cost is a pump, a dose chamber, and controls that have to keep working.

Both add a pump to a system that was otherwise passive, and a pump is a thing that fails. That ties this work to the sewage lift station guide, where the wet well, the duplex pump, the float controls, and the high-level alarm are covered in depth, because a dose chamber or mound pump shares the same failure modes and the same need for an alarm that warns before the chamber overflows.

How far does a septic system have to be from a well?

Setbacks exist to keep wastewater away from drinking water, and they are not the place to improvise. The numbers most often cited are 50 ft from a septic tank to a private well and 100 ft from the drainfield to a well, but the real distances vary widely by jurisdiction, by soil, and by aquifer sensitivity, running from 50 ft to 150 ft or more for the field, and farther for public water supply wells. A septic system that contaminates a well is the failure that makes people sick, which is why this is the hedge to take most seriously.

Wells are only one of the setbacks. The code also sets minimum distances from the system to property lines, the house and other buildings, surface water like streams, ponds, and wetlands, and to any other water line on the site, with the tank and the field often carrying different distances. The drainfield usually needs more separation than the tank, because the field is where treated effluent meets the ground. On a small lot the setbacks, not the soil, are often what decide whether a system fits at all.

Get the setback numbers from the local health code and confirm them with the AHJ before the field is laid out, because they are enforceable distances and they do not bend for a tight site. This is one to verify, never to estimate from memory, because the cost of getting it wrong is a contaminated well, a failed inspection, and a system that may have to be torn out and moved.

The permit, the design approval, and the inspection

A septic system is a permitted, inspected installation, and it is not a job to do quietly without one. The local health department or environmental agency reviews the soil evaluation, approves the design, issues the permit, and inspects the install before it gets covered. The permit is tied to the design that was approved, which is tied to the soil that was evaluated, so the chain runs soil eval, then design, then permit, then build, then inspection, in that order, every time.

The inspection is where the part that matters gets verified, because once the field is backfilled, nobody can see whether the elevations, the separation to the limiting layer, the setbacks, and the components were right. Inspectors check the tank is set and level, the field is at the right depth and the right size, the separation to bedrock or water table is what the design called for, the setbacks are met, and the components match the approved plan. Cover it before the inspection and you may be digging it back up.

This is the hedge that does not soften. Who issues the permit, what the design must show, who is licensed to install, and what gets inspected are all set by the AHJ and the health code, and they are enforceable. An unpermitted system is a liability at sale, a hazard to the groundwater, and in many places an illegal install that has to be brought into compliance. Permit it, get the design approved, and pass the inspection before backfill.

Installing the system without ruining the field

The install follows the approved design, and elevations are the thing to obsess over. The tank is set on a stable, level bed so it does not settle or tip, with the inlet higher than the outlet by the small built-in drop, and bedded and backfilled to the manufacturer's instructions so a plastic tank does not float or a concrete one does not crack. The D-box is set dead level. The field trenches are cut to the exact depth the design calls for, because a few inches too deep eats into the separation from the water table and a few inches too shallow loses soil cover.

The field is excavated in dry, workable soil, never wet. Digging or driving in wet soil smears and compacts the trench surface, sealing the infiltrative face before the system ever sees effluent, and that damage cannot be undone by anything you do afterward. Keep equipment off the field footprint as much as the work allows, work from the side, and place gravel or chambers without letting a loaded bucket ride over the trench bottom.

When it is done and inspected, draw the as-built. The as-built records where the tank, the D-box, and every trench actually ended up, which matters because the next person to pump, repair, or extend the system will be probing blind without it. The design shows the intent. The as-built shows what is in the ground, and on a buried system that nobody can see, the as-built is the only map there is.

Do not compact or build over the drainfield

The fastest way to kill a working septic system is to compact the field, and it is the failure that gets done by accident after the install is long finished. The treatment depends on open soil structure with air in it, and that structure crushes under load. Park a truck on the field, store equipment on it, build a shed or a deck over it, run a driveway across it, or pour a slab, and you have collapsed the soil's ability to accept and treat water. The system was fine until someone drove on it.

Roots are the slower version of the same problem. Trees and aggressive shrubs send roots toward the moisture and nutrients in the field, and the roots invade the trenches and pipe and clog them over years. Keep trees well back from the field, and keep the field in grass, which protects the soil and pulls water without invading.

Tell the homeowner, in plain terms, and put it in the record. No driving, no parking, no building, no paving over the field or the reserve, and keep the big plants away. This is the single most common cause of a premature failure, and it is entirely preventable, which makes it the most frustrating one to find. The soil under that field is the treatment plant. Nobody parks a truck on a treatment plant.

Why do septic systems fail?

Septic systems fail in a short list of ways, and almost all of them end at the same place: the drainfield soil stops accepting water and effluent surfaces or backs up. The causes rank by how often they show up.

Hydraulic overload is sending more water than the soil can take, whether from oversized water use, a leaking fixture, a plumbing cross-connection dumping roof or footing drains into the system, or a field undersized for the real flow. The soil never gets to drain and re-aerate, stays saturated, and quits. Solids and grease reaching the field are next, and they come from a tank that was never pumped or a system with no effluent filter, where escaping solids plug the soil at the trench. Compaction from driving, parking, or building over the field crushes the structure. Roots invade and clog. And plain age catches up, because every field has a finite life even when it is treated well.

Two patterns hide behind those causes. One is no maintenance: a tank that is never pumped fills with solids that then carry to the field, so skipping the pump-out is a slow way of ruining the field. The other is overload that nobody connected to a cause, where the system was marginal and one more bathroom or one downspout tied into the wrong line pushed it over. Catch either early and it is a fix. Ignore it and it is a new field.

How often should a septic tank be pumped?

A household septic tank is typically pumped every 3 to 5 years, and the interval depends on the tank size, the number of people in the house, and how much water and solids go down the drains. The reason to pump is not that the tank is full of liquid, it is always full of liquid, but that the sludge and scum layers grow until they are thick enough to start carrying solids out to the field. A tank pumped on schedule keeps the field safe. A tank never pumped is a field-killer on a timer.

Pumping is not the whole of maintenance. Clean the effluent filter on a schedule, commonly every 6 to 12 months or at each pumping, because a clogged filter slows the house drains and an absent one lets solids reach the field. Watch what goes down the drain: no grease poured down the kitchen sink, no wipes, no flushable anything, no paint or solvents or harsh chemicals that kill the bacteria the system runs on, and no garbage-disposal habit that doubles the solids load. Have the system inspected on the cadence the code requires, which is often more frequent and stricter for ATUs and other alternative systems with pumps and controls.

The schedule is the lifecycle. A conventional system that gets pumped, gets its filter cleaned, and is not overloaded or compacted can run for decades. The exact pumping interval, the inspection frequency, and any required service contract for an alternative system are set by the health code and the manufacturer, so confirm them rather than assuming the 3-to-5-year rule covers every system.

Signs a septic system is failing

Septic failure announces itself, and catching it early is the difference between a pump-out and a new field. The first signs are inside: drains all over the house running slow at once, toilets gurgling, and backups that hit the lowest fixtures first. One slow drain is a clog. Every drain slow together points at the tank or the field, not the trap.

Outside, the field tells on itself. A patch of grass over the field that is greener, lusher, and wetter than the rest of the yard means effluent is rising close to the surface. Soggy ground, standing water, or actual sewage surfacing over the field is the field failing to accept water, and a sewage odor in the yard goes with it. Effluent at the surface is a health hazard and a clear sign the soil's capacity has been exceeded.

When the signs show, get the tank checked and pumped first, because a tank full of solids or a clogged filter is the cheap, common cause and ruling it out costs little. If the field itself is saturated and surfacing, that is a bigger problem and a call for the installer or the health department, because a failed field usually means replacement in the reserve area. Either way, sewage on the ground is not something to watch and see, it is something to act on.

What to keep on file

A septic system is buried and invisible, so the record is the only way anyone knows what is down there and how it has been cared for. Keep the soil evaluation and perc results, the approved design, the permit, the as-built showing where everything actually sits, and a pumping and maintenance log with dates. That file answers the questions that come up at every pump-out, every repair, and every sale.

The pumping log earns its keep more than people expect. It proves the tank has been maintained, it sets the interval for the next pump by showing how fast the sludge builds, and it is the document a buyer's inspector asks for. A system with a clean maintenance history sells. A system with no records reads as neglected whether it was or not.

A field tool like FieldOS is a sensible home for this, because it ties the soil eval, the permit, the as-built drawing and photos, and the recurring pumping and filter-cleaning schedule to the property, so the record follows the system instead of living in a drawer that gets thrown out when the house changes hands. The point is durable, findable records on an asset nobody can see, kept where the next service call will actually look.

Common mistakes

• Skipping the soil evaluation or designing a system for a site that cannot treat wastewater.
• Undersizing the tank or the field for the home's real flow.
• Installing no effluent filter, so solids reach and clog the field.
• Compacting, paving, or building over the drainfield or the reserve area.
• Hydraulic overload from leaks, oversized use, or stormwater piped into the system.
• Never pumping the tank, so accumulated solids carry out to the field.
• Ignoring the setbacks to wells and surface water, contaminating drinking water.

Field checklist

Standards and references

Septic and onsite wastewater systems are regulated at the state, tribal, and local level, not by the federal government, so the controlling document is the local health code or onsite-wastewater regulation and the permit it requires. That code sets the soil evaluation procedure, the design flows, the loading rates, the tank and field sizing, the setbacks, the materials, and the inspection. The EPA publishes onsite wastewater guidance and design manuals that inform many of those state codes, but the EPA does not permit your system. The AHJ does.

The people who carry the authority on a project are the licensed soil evaluator or professional engineer, who performs the soil work and signs the design, and the health department or environmental agency that approves it and inspects the install. Anything in this guide about soil, sizing, setbacks, or permitting is the general shape of the work. The binding numbers come from your code and your soil evaluator, confirmed with the AHJ.

Three things carry the whole job. The soil evaluation comes first and the soil is the treatment, so feasibility and every design number trace back to it. The system is sized to the flow and the field is protected with an effluent filter and no compaction, because the field is the part that fails and the part you cannot easily replace. And it gets permitted, the setbacks get met, and the tank gets pumped on schedule, because those are the steps that protect drinking water and keep the system alive. For the cases that sit next to septic, see the sewage lift station guide where the waste has to be pumped and the building sewer lateral guide where a public sewer takes the place of the field.

Units and terms

Septic work carries its own vocabulary, and the same component goes by more than one name across a code book, a soil report, and a supply house.

Flow is in gallons per day (gpd) and design flow is set per bedroom. The perc rate is in minutes per inch, and the soil loading rate is in gallons per square foot per day. Field area is in square feet. Setbacks are in feet. Knowing which unit a number is in keeps a loading rate from getting confused with a perc rate, and keeps a design flow from getting read as a tank size.

Septic system

Onsite wastewater system that treats sewage on the property where there is no public sewer, using a tank and a soil drainfield

Drainfield / leach field

The bed of trenches in native soil where effluent is dispersed and the soil treats it; also called the soil absorption field

Percolation / soil evaluation

The perc test measures how fast soil drains in minutes per inch; the soil evaluation reads texture, limiting layer, and water table to set feasibility and design

Septic tank, scum, sludge

Buried watertight tank that settles solids (sludge, on the bottom) and floats grease (scum, on top), passing clarified effluent to the field

Effluent filter

Screened cartridge in the tank outlet that catches fine solids before they reach and clog the field; cleaned on a schedule

Distribution box (D-box)

Buried box that splits effluent evenly among the drainfield trenches; must be and stay level

ATU / mound / pressure dosing

Alternative methods for poor sites: an aerobic treatment unit adds oxygen for higher treatment, a mound builds the field up in sand, pressure dosing pumps even timed doses to the field

Setback

Minimum code distance from the system to wells, surface water, property lines, and buildings to protect drinking water

Reserve area

Suitable soil set aside and protected for the future replacement field, required by many codes

Related tools

Calculators and readiness checks for this work

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FAQ

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• What is a building sewer?
• What is an intrusion alarm system?
• What pipe is used for compressed air?
• When do you need a fire pump?