This article is principally for homeowners in Rhode Island who are building a new home or looking to have a septic system designed. We perform system designs (Title V) in Massachusetts, but MA predominantly uses conventional septic systems with some rudimentary treatment designs (e.g. Presby systems). Advanced treatment systems are generally still in the experimental or piloting stage there. In MA, DEP is the controlling agency but day to day applications go through local Boards of Health so designs can often be dependent on the Municipalities’ requirements. RI tends to be more complex, but is a centralized system; all applications go through Rhode Island Department of Environmental Management (RIDEM)

***edit 08.09.19: DEM has released a septic page just for homeowners: Good info for those in need, see: ***

Septic systems are not sexy. Very few clients come in and rave about our septic design capabilities (as impressive as they are!). Most people just want it to be done – unseen, inexpensive and quickly. We can understand. If I’m being frank, I wish all of our clients were on a sewer system. Generally (if capacity is sufficient), these can provide better overall treatment, at a lower cost. We will avoid the politics of this discussion (which can be a hot button issue, for rural and suburban towns concerned about urbanization due to sewers) but just note that septic systems are generally an “inferior” system, as they are decentralized and hard to manage. Treatment designs have improved dramatically in recent decades but in practice we have a long way to go to get all older system types removed in favor of modern designs. While the states are making strides in this direction, there is still a lot of work remaining.

To us at NEI, getting the right septic system for the site, and more importantly, designing the septic system as a component of the whole is key. Generally we excel at full service design (from surveys and testing, to site, drainage, system design and to building and structural design). This allows us not only to pick the right system but to make sure the design works with all aspects of the site. Coordination becomes easy, as our team has worked together on hundreds of designs. If you’re using multiple firms / professionals, make sure they coordinate!

Let’s go through the basics of an application to RIDEM On-Site Wastewater Treatment Systems (OWTS). I want to look at this from two perspectives, one being the homeowner and the other being RIDEM. Residential Septic systems are sized by the number of bedrooms. That’s it. You can have 87 bathrooms, a 10,000 sq.ft. home and be rated as a four bedroom house if that’s the case. Each bedroom, by the book, adds 115 gallons per day of wastewater flow to the ether. A bedroom is any enclosed / private room (with a door) larger than 80 sq.ft. There are many definitions, but DEM’s is generally the most restrictive and prevails. Often, the local building inspector makes the determination for specific rooms, if they are a bedroom or not. The most common litmus is the private door. A room with a cased opening (no door) is generally not a bedroom. A room with one is (even if you try to call it an office, study, exercise room, etc.). Remember, DEM and the municipal officials see hundreds of these every year. They know the tricks; don’t think you’ll pull one over. Commercial systems are sized by flow, based on use table in the RIDEM OWTS regulations. We’re not going to get into that much, but it’s a logical process based on historical water record data.

That being said, if you’re in need of a septic design then it’s usually for one of the following reasons:

  • You are building a new house in an area without sewers and need an OWTS design as part of the planning process. This is called a New Construction Application. You are increasing. For the homeowner, this is more or less a nuisance. It’s not the focus of the project and you just want to get it done and installed as inexpensively and unobtrusively as possible.

From DEM’s perspective, you are increasing overall wastewater flow by building a new house and adding bedrooms. This adds wastewater and its constituents (nitrogen, phosphorous, etc.) to the environment. The environment is generally at or above its natural carrying capacity and each milligram of pollutants can add to problems like eutrophication (removing oxygen from the water), which leads to fish kills, red tide, surface water and drinking water contamination (the water you and I drink!).

With that in mind, septic systems are important as they treat the waste on site and keep these pollutants down to a minimal and reasonable level. RI is a small state and effluent from northerly parts of the state will make its way to Narragansett Bay and impact coastal water. The systems are connected! Many coastal areas and ponds are heavily impaired – septic effluent is a major contributor.

  • Another type is an Alteration application. This is generally adding one bedroom only (usually as an addition) and the process is similar to a New Construction application.
  • Also there are Repair applications because a system failed or has been declared a cesspool (which need to be removed by 2023* becuase they are being phased out). Contrary to its name, we don’t actually repair the system in the ground – it’s almost always removed and replaced with a new modern system in full.

The key element of a repair application is that you are not increasing flow. In this case, the OWTS regulations can be slightly more lenient as you’re not adding additional flow and improving the baseline condition. Often they will grant allowances to setback distances and similar restrictions.

*For exact dates, please refer to RI Cesspool Phase-out Act FAQ’s

Generally, system installers (excavators) can still design some repair systems (Class 1 Licence), but you need to be very cautious in these cases. We NEI (and most other design firms) will incur higher design costs but it’s also a different product. Excavator’s designs are rough, hand drawn – it’s more of a design build process whereas NEI provides actual engineered drawings. Design by the excavators is becoming rarer. Class 2 Designers can install conventional / smaller systems done by professionals (Engineers and Surveyors). For any advanced treatment design, you need a Class 3 Engineer. Make sure you are using the appropriate professional(s) for you needs. Don’t be afraid to ask about licenses and confirm them yourself.

Ok, so we’ve discussed application types and who can do what; perhaps you’re reading a proposal from an engineering company for septic design services. Let’s go through the finer points:

Generally, my line to clients is that we want to design you the least expensive, least invasive system that conforms to DEM Regulations. That’s the 30,000 ft view. How we do that depends on a few factors that are determined by a topographical site survey (NEI item 1.5) and a Soil Evaluation by a Class IV soil scientist (NEI item 3.2).

For the soil evaluation, we subcontract an excavating company and dig at least two test holes in your yard. We’re doing this to determine:

Water table depth: This is key, as it helps to determine at what elevation your system can be placed (e.g. how high it is, which determines how much fill needs to be brought in). The soil evaluator is basically looking for a “rust line” like the discoloration of a seawall from the high tide. The iron in soil is reduced and produces redoxomorphic features that can be viewed (generally an orange or reddish ‘mottle’). In rare cases, the soil could be disturbed, or very dark, and we will have to physically measure the water table over the wet season (Jan – April), which is why we install the PVC pipes in the hole.

If you’re proceeding with a New Construction application, you need to have at least an 18” to 24” water table to install a system without variances (which are risky and not a guaranteed success). Sites with water tables (two tests holes) above 18” to 24” can be built “by right” meaning that as long as the OWTS rules are followed, you will receive an approval from DEM.

Generally speaking in RI / southeast MA, soil evaluations that yield water tables better than 4’ are considered to be “good” results and water tables below 4’ (to 2’) are marginal. 2’ and below are poor results. Anything in the 12” to 17” range might be buildable but are subject to a variance review by DEM. These applications are expensive and time consuming. Anything less that 12” is not feasible for a septic system. These soils are generally considered “hydric” soils (wet, very often).

Soil Category: This is how dense or compact the soil is, as well as its constituency. Sandy and gravely soils allow water (or wastewater) to be infiltrated at a faster rate. Denser, silty soils infiltrate at a lower rate. This ultimately controls the necessary size of the system. Our soils are often found at the edge of the scale. Low lying neighborhoods formed by glacial outwash will often have “good” soils that are stratified sands and gravels, whereas many higher areas have soils formed by glacial till that are generally “poor” due to a high water table and poor infiltration rates.

Ok, so now we (hopefully) understand the basic about soils and application types. How do we design a system?

Again, our goal is to build the lowest cost system, conform with regulations and minimize how invasive the system is (e.g. grading is required, as well as septic covers, vent pipes, maintenance requirements, etc.). Septic systems need to be carefully sited, away from water lines, wells, property limits, buildings and similar features. The elements are key to the proper function of the system and conformance with state standards.

We can break the design process into two general categories: Conventional Systems and Advanced Treatment Systems.


Conventional Septic Systems work by gravity, these are the “old school” systems and generally appropriate for larger sites (1+ acre) with a “good” water table (4’+). They are composed of a Septic Tank, Distribution Box and Leaching Field – very simple. Usually, you just need to do a septic tank cleaning with the help of a septic tank cleaning service at 6 to 12 month intervals, and consider septic tank pumping every 2 to 5 years…so these septic systems are lower installation cost and lower maintenance cost.

Advanced Treatment Septic Systems are newer (though they have generally been around for ~20 years and are well tested and reliable) types of systems. These have a Septic Tank but then use a treatment system (like a miniature sewer treatment plant) and a recalculating pump to treat the effluent and minimize waste strength (generally including Nitrogen reduction) before going to the Leaching Field. These require a maintenance contract, pumps and bi-annual maintenance. More expensive to design and install but work for more critical sites (small sites, sites near wetlands or salt marshes / coastal wetlands and high water table sites). They’re also more expensive to maintain and require a maintenance contract which needs to be recorded.

List of approved advanced systems:

General Hierarchy of design (in order of complexity and preference) Meaning: NEI will always try to install the lowest cost system requiring the least disturbance that will comply with State, Municipal and applicable regulations and is suitable for the site and use.

Conventional: All below ground with a grass surface, typically. These septic systems may cost anywhere from the mid teens to low twenty thousand range. Costs are highly variable, and must be obtained on an individual basis (generally costs referenced are for 3 to 4 bedroom systems). We prioritize these septic systems because of their simplicity, low cost and ease of maintenance. Many factors influence costs and design though – this is a general guide only. Septic systems typically increase in complexity, in design and cost of installation, the further you go down the list.

Trench Systems: The oldest of the “old school.” These consist of a pipe bed in crushed stone and use leaching trenches (“fingers”) to accept effluent. It’s becoming very rare to design these systems, as most of the upland parcels have already been built upon. They still work for sandy / gravelly sites on large tracts.

Diffuser Systems: Basically a large concrete chamber for effluent distribution. Inexpensive and durable (you can drive over them and pave 25% of the area above them). These are very large, cost effective and can only be used with water tables of at least 4’. Although they really only work well at sites with water tables of 6’+.

Eljen Systems: (eljin, eljen) Technically proprietary / advanced treatment, but it’s a modern take on a trench system. Instead of stone, it uses a plastic core which promotes biomat formation and effluent treatment. Generally used on larger sites with poorer soils (but with water tables 2’+)

Advanced Treatment:

Pressurized Shallow Narrow Drainfield (PSND): These use a pressurized distribution system, enclosed in a half pipe PVC shell with and media over the natural soils for treatment. As the name implies, these are very shallow with a grass surface. This kind needs to be protected from traffic as they can be easily damaged. These use an advanced treatment system** for pretreatment.

Geomat systems: Very similar to the PSND, but with a proprietary leach system that is easy to install. Like the PSND, these use advanced treatment (which is a treatment pod about 5’x7’ in size with a green cover to grade), 3 green covers (for the septic tank and pump), and are set to grade with a shallow pressurized leaching field. These generally have a grass cover, to grade.

Bottomless Sand Filter (BSF): Infamously called a “sandbox”, these are raised systems, from 6” to 3’+ above grade. These use advanced treatment and are required in areas where the water table is 18” to 24 by regulation (meaning, you have no choice but to use this type of system). These provide a very high level of treatment in a very compact area. That being said, they are generally unsightly. We usually try to design them so that they are situated in a corner or down a hill so they can be screened with decorative planting and not be seen from the home or patio areas.

The preceding are the most common types of systems encountered but there are some “odd” systems (composting toilets, etc.) that can be used in very rare cases.

Advanced treatment systems cost more to design, as they are more complex. They generally cost more to install as well. A typical 4 bedroom system starts around the mid twenty thousands and can cost more for higher capacities or complexity.

Certain areas that have particularly high pollutant loading or are environmentally sensitive may require advanced treatment design, even if there are good soils on site. Areas like Touissett (Warren), Jamestown Shores, Island Park (Portsmouth), areas with drinking water wells and the South County salt ponds (among others) may have extra design restrictions. DEM maintains additional design regulations for many critical areas like these.

**There are many advanced treatment systems approved by DEM (Orenco, FAST, Septi-tech, etc.). Often, these all have their strong points and weak points regarding cost, treatment, installation difficulty, etc. Generally, we use the Orenco Advantex line of systems. We find these to be cost effective and very reliable. There is a good installer base (competitive pricing), multiple dealers and robust technical know-how and assistance in RI. There are other systems that are used from time to time; it all depends on the site and design specifics.

These systems usually have multiple pumps, control panel, remote monitoring and maintenance contracts. Once approved, you will need to record the Operations and Maintenance (O+M) in land evidence records.

Ok, now that you’ve understood and signed your contract, what happens next? Usually we will perform the site survey (topography, and metes and bounds / property line survey if the system or site components are near the property limits), and soil evaluations. Then we will setup a conceptual design (using your building and / or architectural design plans) to create a proposed site plan. Generally, we will review a draft design with the client(s) to make sure they understand the design. Once that’s complete, we will submit to DEM. Applications generally take 4-8 weeks, excepting variances which take much longer. Simple repair applications often take less time than advanced treatment / new construction designs.

Once your design is approved, you will be mailed approved copies (be sure to keep these because approved plans must remain on site during construction). From there you can select a contractor and start moving.

Key points in selecting a contractor:

  • Licenses: Make sure they have the right license, both for conventional and advanced treatment systems (where needed). Make sure someone in the firm has the licenses – be wary of hiring installers using a “friends” license. These can be found here:
  • Insurance: Fairly obvious, ask for proof of insurance (generally a Certificate of Insurance (COI).
  • Experience: Don’t be afraid to ask for references and call them or visit those sites. You wouldn’t buy a car without test driving it; the septic may cost as much (or more)! Do your homework. Check to make sure the contractor has no disciplinary actions taken.
  • Timing: If you have a deadline, get it in the contract – otherwise it may not be met.


NEI (or our staff or subcontractors) will be responsible for construction oversight of the system. Depending on the complexity, DEM will require inspections at certain stages of construction. These are typically:

  • Bottom inspections (after the field has been excavated to make sure the soils are as expected)
  • Cover inspection of all components prior to backfilling (covering) the system
  • Advanced Treatment system inspections
  • Joint applications, with RIDEM / CRMC
  • Verification of Site and Erosion Control.

Your designer will work with you and your system installer to facilitate these inspections, as noted on the approval form. We require the installer provide their telephone, email, company info and licensee information, prior to inspections. All inspections go through our main office and require notice, as outlined in our proposals.

Once the installation is finished, you may be required to sign and provide operations and maintenance documents (Advance treatment systems) and your installer may be required to provide material slips. NEI, after final inspections will submit a Certificate of Conformance to DEM. Once received, you will be mailed a copy. This is verification that the septic system is installed in substantial conformance with the design plans and regulations.

From there, you can finally, um, use the facilities…

Phew. That’s quite the process, but NEI and our sub consultants are well versed in these procedures and we can guide you as much (or as little) as you need. We want to make sure your site design and construction process goes as seamlessly as possible.

Side Note: Putting in a pool with a septic system.

So, you can have an inground pool with a septic system, provided the correct setbacks are adhered to. In RI, that means the pool needs to be at least 25′ from the septic field, and that all of the sub-components (Septic: Septic Tank, D’Box, Pumps, lines etc. vs. Pool: pumps, heater, filter, lines, etc) need to maintain sufficient separation. Designing new houses with Pool and a septic is fairly easy, as long as room exists on site.

Trying to design a pool to work with an underground septic that’s already been installed is a little trickier. In many or most cases, formal As-Builts are not prepared for Septic Designs (though, we do recommend it!!). There are typically field sketches done, but these are often “rough” notes an not to be relied upon without verification. So, the first step, in this case, is trying to find exactly where the septic system actually is! If you have an as-built plan, great, we can use that. If not, we need to literally do some digging, and subcontract an excavation company to dig around the corners of the system and have our surveyor physically locate them on a plan, so that a new design can be done. Once we know the actual extents of the pool, the design becomes much easier, but be prepared for some cost and time if field verification/excavation is needed.

Typical OWTS Process: 

  • Survey – locate boundaries, utilities, identify constraints
  • Identify suitable locations for soils testing
  • Identify if soil test needs to be witness by DEM, if so, file SEV application. If not, conduct SEV
  • Subcontract excavator (least cost / best availability)
  • Dig Safe
  • Conduct soil evaluation
  • Identify suitable type of systems, review with owner
  • Plan out additions, new build, etc
  • Review with owner and sign applications
  • Prepare System Design + App.
  • Once complete, submit to RIDEM for review (under requisite app, repair, alteration, new construction)
  • Comment / Review Cycle with DEM  (as needed)
  • RIDEM Permit Approval
  • Put plans out to bid, obtain pricing, sign contract, etc
  • Inspect per approval (Cover Bottom, Advanced Treatment, etc)
  • As built
  • Record OM, if advanced treatment
  • System Conformance

Helpful links:

  1. OWTS / RIDEM home:
  2. RI Wastewater Fact Sheets:
  3. How to hire a contractor: 
  4. New Home application info:
  5. Repair application info:
  6. Does your system meet state standards:
  7. Septic system checkup:


This article is not a substitute for professional consulting by an Engineer or Surveyor. This blog contains general design information and is intended to serve as guidance for homeowners. Certain instances or design requirements may contradict these statements. Every site is different and requires customized design by a licensed professional!

Neal Hingorany

Get in touch with us for more info on septic systems.