Before You Buy Land Think Of The Home’s Sewage Disposal System


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Keep Sewage  System Treatment in Mind when Buying Vacant Rural Lots

Are you searching for the perfect patch of rural land on which to build the house of your dreams? If so, you will also want to consider more than just the views and the neighbourhood before signing the purchase contract.

One of the most important things to consider when purchasing vacant lots outside city infrastructure is whether there is an onsite sewage treatment system available or whether you need to have one installed. As many local authorities will not issue a building permit before your onsite sewage system has been approved by the health department, it is important to consider this well in advance.

If you need to install an onsite sewage treatment system, you will need to carefully consider this when purchasing your piece of paradise, as site and soil conditions will determine what type of system is appropriate and where you can you can install the private sewage disposal system. This may, in turn, affect your overall construction budget, as well as limit the amount of space left for you to build your home. Here’s what you need to know…

sewage system

Buying Unserviced Land

If the lot is not serviced by a municipal sewage treatment system, the wastewater generated by the home will need to be treated onsite, in which case an onsite sewage treatment system or septic system will need to be installed.

A conventional septic system typically consists of a pipe that channels wastewater from the home, a large septic tank (concrete, plastic or fibreglass holding tank), and a gravel-filled drain field where the partially treated wastewater is discharged and undergoes further treatment as it passes through the soil. However, depending on site conditions, a more elaborate system may need to be installed, which could increase costs…

Sizing A Sewage Disposal System

One of the most important things to take into account when designing a private sewage disposal system is to make sure it appropriately sized. The size of the system will depend on the size of the home and the number of occupants and is usually calculated according to the number of bedrooms (i.e. potential occupants).

However, other factors, for example, the number of water-hungry appliances such as dishwashers, also need to be factored in. A large six bedroom home will require a much larger onsite sewage system than a smaller two or three bedrooms home, and the footprint that both the sewage system and the home will take up on the lot needs to be considered.

Also, you need to allow for space to install a second onsite sewage system in the event that the initial sewage system fails and needs to be repaired or replaced. If you are planning on building a large home on a small lot, the space required for a septic system (and possibly also a second backup system) could be a limiting factor that could restrict not only the placing of the home on the plot, but ultimately also the size of the building that you can feasibly construct on the plot.

The soil characteristics will also affect the size and type of the sewage system needed. If the soil on site is dense and not sufficiently porous, wastewater will not be readily absorbed. In this case, a larger drain field is required. It is preferable to have a system that is capable of processing more wastewater than what your home generates rather than the other way round.

This allows for visitors, in addition to the family, or periods of exceptionally high rainfall that could result in a higher water table and the soil becoming saturated. If the discharged liquid is not readily absorbed by the soil, you run the risk of sewage backing up into your home or pooling on the surface, where it can contaminate surrounding groundwater or surface water sources. More info on how soil and site conditions affect an onsite sewage treatment system design can be found below.

Soils & Restrictive Elements In Sewage System Designing

An onsite sewage treatment system can only be installed in an area where the soils allow the discharged liquid effluent to percolate into the ground below. While the soil on some sites may allow a private sewage system to be installed anywhere, other sites may be much more limited, or may not have any area that is suitable for a conventional drain field at all.

It is therefore important that anyone wishing to buy vacant, unserviced land, determines whether the site can support an onsite sewage treatment system, and if so, where the sewage system needs to be located. A site evaluation may reveal that the only suitable spot to locate the septic system is right where you had planned on building your house.

Soil Testing For Septic

Besides having good percolation, for the sewage system to function efficiently, there needs to be sufficient soil depth between the drain field and any underlying restrictive layer, such as a high water table, underground river or spring that can become contaminated, or impervious bedrock or layer of dense clay or silt that restricts absorption. However, coarse sandy soils that are extremely porous may also be unsuitable. They drain too quickly, and as a result, the effluent is not effectively treated by bacteria in the soil before it reaches the water table.

A rocky site that has only a shallow layer of topsoil between the surface and underlying bedrock may make it impractical to install a conventional septic system. Firstly, there may not be sufficient depth to bury a conventional septic tank, in which case a shallower septic tank with a larger surface area or footprint may be a suitable alternative option. More importantly, a drain field installed in a shallow layer of soil is not likely to perform well and it is unlikely that local health officials will approve it.

If you have your heart set on a specific vacant unserviced lot that has a high water table or poor soil conditions, making it unsuitable for installing a conventional septic system, all is not lost. There are ways of working around these issues by installing a non-conventional ‘alternative sewage system’, such as an aerobic treatment unit (ATU) or an above ground sand mound system — where more favourable conditions are artificially created by installing a mound of more suitable soil above ground level, which then serves as the drain field. But as they can prove to be more expensive than a conventional sewage system, it is good to know what you are in for before you commit to purchasing the property.


Another important thing to consider is the topography of the site. If the lot is located on a steep slope, it may pose problems for installing both the sewage drain line leading to the septic tank and/or the drain field effluent piping. The pitch of both the sewage drain line pipe and the drain field effluent piping is important, as it can determine whether the system works efficiently or not.  An alternative sewage system or additional materials may have to be used in such cases of more extreme sloped topographies.  

topography for septic systems

A pressure distribution may have to be implemented to direct the wastewater flow to a suitable area for the final sewage treatment disposal.  This area may be located on higher levels than the construction area of the home.  

Pitch of Drain Line Pipe Leading to Septic Tank

The recommended pitch or slope of the drain waste pipe leading from the home to the septic tank is 1/8″ to 1/4″ of slope per linear foot of pipe, which would normally result in a wastewater flow rate of around 2 feet per second. If wastewater flows through the waste pipe into the septic tank (or sewer) too quickly, it may leave solids behind, which could accumulate and clog up the pipe. On the other hand, if wastewater flows through the waste pipe that leads to the septic tank (or sewer) too slowly — as may happen on a flat site or a site with a very low slope — the sluggish flow may also leave sewage behind to block up the pipe. The pitch needs to be just right.

pitch of drain line

Pitch of Drain Field Effluent Dispersal Piping

By the same token, the effluent piping installed in a gravity fed drain field also needs to be correctly pitched in order for it to effectively treat the partially treated wastewater effluent discharged to it from the septic tank. If the effluent lines are pitched too steeply, the effluent will flow to the lower end of the drain field effluent line too quickly and may accumulate there, overloading it and saturating the soil, causing effluent to pool on the surface. To work around this (i.e. to slow down the flow), on steeply sloped sites, contractors may install drain field effluent pipes parallel to the slope, stepping down gradually as they zig-zag down the slope.

Small Space & Other Constraints For Sewage Systems

If the lot you are interested in is small and you are concerned that the onsite sewage treatment system you need won’t fit on the site, you may find some relief knowing that experienced septic system installers have ways and means of designing a sewage system that will fit and work on a small site where space is limited. The same applies to difficult sites such as rocky sites, sites with poor soils, steep sites and wet sites. Some options for smaller sites include installing a more robust traffic-bearing septic tank under your driveway; and/or a sewage drain field with a smaller footprint, such as a sand mound drain field raised above ground level or a sewage leach field that extends deeper into the soil. Both these options consist of vertical (rather than horizontal) drainage beds that treat wastewater as it passes through.

A sewage system expert may also recommend an aerobic treatment unit or other types of non-conventional wastewater treatment system be installed on sites with space or other constraints (e.g. sites with poor soils/drainage, steep sites, sites with a high water table or other restricting factors). Alternative septic systems can reduce the size of the area needed for the sewage drain field or the soil absorption area significantly, and in some cases eliminate the need for these entirely.

Lots Located Close to Water

Living alongside a river or on a lakeshore property sounds idyllic, and is something many of us dream about. However, if you are wanting to purchase a lot that is situated close to a waterbody, there are additional considerations you need to bear in mind when it comes to onsite sewage treatment. To prevent contaminating the waterbody you look out onto, and which you, your neighbours and possibly also communities further downstream may depend on for a source of drinking water, special attention needs to be given to both the type of sewage disposal system you have installed, as well as the maintenance of your onsite sewage system, which may require more frequent pumping.

As most of the wastewater treatment process takes place in the soil of the sewage drain field, soil conditions and a high water table may make it impractical to install a conventional septic system on a waterfront property. The soil may quickly become saturated during rainy periods when water levels of the adjacent river or lake, rise, which can cause partially treated wastewater to enter and pollute the waterbody. The shoreline may also erode over time, decreasing the distance between your sewage system and the waterbody. Besides the obvious health threats associated with exposure to harmful bacteria and viruses found in wastewater, onsite sewage disposal systems are also a source of nutrients (nitrogen and phosphorus) that can fuel the growth of aquatic algae and weeds, which can cause both an aesthetic and environmental problem. Weeds can rapidly take over the surface of the water, making accessing the water for recreational activities difficult, while algal blooms can strip oxygen from the water when the algae cells die, killing fish and aquatic organisms.

Alternative septic systems such as a sand mound system, sand filter or aerobic treatment unit may offer a cost-effective solution. Bear in mind that some options have pumps which require electricity to operate, which can drive up your electricity bill and may not be suitable if you are planning to purchase the lot to build an off-grid retreat. It would be best to seek advice from your local health authority or Registered Waste Water Practitioner (ROWP) to ascertain what is permitted so you can get a better idea of what your options are. An experienced sewage system contractor can also advise you on the best solution for your needs.

Make Sure You Have Enough Room Left for the Home

There are a number of restrictions governing where an onsite sewage disposal system can be installed on a property. Sewage systems cannot be installed too close to wells or other water sources, and must also be set back from buildings, driveways, roads and property boundary lines. These regulations not only affect where you can install your sewage disposal system but may also affect where on the site you can build your home. You will need to ensure that there is sufficient space to install an appropriate sewage system in a location that is permitted, while still leaving sufficient space to build your dream home. You don’t want to find out after you have signed the deed of sale that the space available after the onsite sewage disposal system has been installed is too small to accommodate the size home you require, and/or the home cannot be built in the location you desire.

Site Evaluation

Once land has been purchased, a septic permit must be obtained from your local authority before an onsite sewage disposal system can be installed. This is typically only issued after a site evaluation has been conducted in some instances the local health authority or an authorized person (ROWP or engineer). A site evaluation usually includes a soil test (including a percolation test) and a topography assessment and will determine whether a conventional septic system can be installed or whether you will need to opt for an alternative septic system. Either an alternative septic system or conventional systems, a sewage system certified practitioner will need to design a sewage system specifically for the conditions found on your site. As alternative septic systems can work out much more expensive than a conventional gravity fed septic system, it’s important to know beforehand so that you can include these costs in your building budget.

Legal Considerations: Protecting Your Interests Within The Purchase Contract

If a site evaluation has not yet been conducted for a property you are interested in purchasing, it would be wise to include a clause in the purchase agreement that the sale is subject to receiving an acceptable site evaluation. This will allow you to negotiate a better price if the site evaluation is not too favourable, or even opt out of the sale agreement entirely. It is advisable to seek the advice of an experienced real estate attorney, who can help draw up a purchase agreement that includes a contingency clause in the contract that the purchase is contingent upon receiving approval from the local authority to build an acceptable onsite sewage disposal system and house on the property. If for some reason, you cannot, the land is essentially worthless.


When buying a vacant lot, one of the most important questions to ask the seller is whether an onsite sewage disposal system site evaluation has been conducted to assess its suitability for wastewater treatment and disposal.

If it has, the next questions to ask should be:

What type of sewage system is permitted on the site? What are the associated costs? And where must the sewage system be located? Knowing the answers to these crucial questions will not only give you a better idea of what you need to budget for in terms of your building costs but also where on the site you can place your house.

Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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