Puget Sound Property Owners Provide Many Interesting Onsite Design Challenges

He used to serve up frosty desserts, but Tom Prudum now designs septic systems for a living — and he loves it

Puget Sound Property Owners Provide Many Interesting Onsite Design Challenges

Tom Purdum oversees an OSCAR septic installation he designed in Edgewood, Washington. The installer is Lewis Gregg of A-Bell Excavating of Puyallup, Washington.

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It turns out it is possible to have too much ice cream. At least that was the case for Tom Purdum.

Prior to joining the onsite wastewater industry, Purdum owned a small-batch ice cream company, operating two stores and an ice cream truck. It was a suggestion from his brother that led him to put down the scoop and pick up a career in septic design.

Purdum’s brother was already a designer and offered to teach him the ropes. “I got burnt out of the ice cream business and thought it was time for a change,” Purdum says. “So, I told my brother I was in.”

Purdum sold his house in Spokane in eastern Washington, and moved west to the Puget Sound area near Gig Harbor, where he apprenticed for four years under his brother. “He actually brought me in as a partner in his business which was very generous, and I got an excellent education,” Purdum recalls.

After completing the apprenticeship and a few years in business together, they went their separate business ways and Purdum started American Septic Design in 2022. Operating as a one-man show, Purdum is on track to design roughly 120 systems per year in the Seattle area.


As a new designer, Purdum says he doesn’t yet have the name recognition and word of mouth referrals to depend on. Because of this, he expands his service area to cover primarily Pierce, Kitsap, Mason and a little bit in Thurston and King counties. It’s a territory that includes many communities within the Puget Sound watershed.

“Everywhere you go here is a shoreline almost,” he says. “And it’s not only shorelines, it’s just wetlands everywhere.”

The wetlands create some variations in county codes, and Purdum says counties with the most wetlands and critical areas tend to tighten up regulations quite a bit. “Washington septic designers are licensed to work anywhere in the state,” Purdum says. “But every county has its own additions to the state code. Some counties are more lax on wetland regulations while others, like Pierce County, are very stringent. Learning each county is challenging to say the least.”

Though wetlands are prevalent, the area presents a variety of site conditions. “The terrain here is varied,” Purdum says. “Just outside of Gig Harbor and the Key Peninsula area, it’s very wooded and has some decent elevation in the terrain. And then when you get into East Pierce County, it’s flat farmland.”

For the most part, Purdum is working with what the state calls Type 4 soils, which is loamy fine sand or fine sand. “There are also some areas that have a medium sand which we love to find, and a few locations with coarse sand,” he says.


To Purdum, wet terrain is not the most challenging part of designing in the area. Property size is.

“The biggest issue we come up against is lot size,” Purdum says. “There are a lot of small lots that don’t meet the minimum lot size for Washington state that was created after a lot of these lots were put in, and that can create some problems.”

Anderson Island is a great example and located in the heart of Purdum’s coverage area. There is a plot on the island called the Riviera, which is made up of lots from 8,000 to 10,000 square feet, while the minimum lot size now for the state is 12,500 square feet.

“So what that means is, you can have a 9,000-square-foot lot and you can still put a septic on it as long as we can get something to fit,” Purdum says. “However, we can’t ask for any sort of waivers or variances from the code.” Washington state code says variances can be requested for lots over 12,500 square feet.

“I had a waterfront lot project where someone had cut a driveway through the lot at an angle and really destroyed the usable drainfield area for a three-bedroom home,” Purdam says. “I had to fill out a waiver with the county to reduce it to a two-bedroom drainfield and that was acceptable because we had enough square footage in the lot. If it had been under 12,500 square feet, that driveway could have potentially made that lot unbuildable.”

With the number of small lots he works on, multiple design options are a luxury Purdum often doesn’t have. “A lot of times I don’t have any flexibility,” he says. “There is only one spot a drainfield can go, so I dig the test holes and go from there.”


The abundance of high water tables and small properties has led Purdum to rely on a few specific system designs and products, which for the most part, he sources locally.

He commonly uses precast concrete tanks made by either Hagerman Pre-Cast out of Poulsbo, Washington, or Evergreen Pre-Cast located in Sumner, Washington. When a job calls for something else, Purdum also relies on Infiltrator and Roth tanks.

Another tool in his belt for wetland design is OSCAR systems, a short-sand mound and coil distribution system from Lowridge Onsite Technologies, another Washington-based company. 

“They are a great alternative to regular mound systems,” Purdum says. “It’s six inches of C-33 sand with drip irrigation tubing arranged in a coil system on top of the sand. Multiple coils are arranged in groups to form a lateral, and six more inches of sand is put on top of the coils making a sort of trapezoidal mound.”

To provide the treatment, Purdum says really any ATU system that meets the requirements of the OSCAR systems works fine and the end result is a low mound that can be feathered into the landscape.

“You hardly even know they are there,” Purdum says. “And the nice thing about them is you can configure them really any way you need to. You can bend them around corners, put them around trees and as long as you stay level with the topography you’re fine.”

Orenco pumps and SJE Rhombus control panels are commonly incorporated into his designs as well.


The need for more housing due to the rising population and shortages of places to live has had a large impact on the way Purdum designs.

The idea of accessory dwelling units isn’t new, but encouragement from the local government to build them is. “A lot of municipalities are pushing for high-density housing and where ADUs used to be frowned upon a while ago, now we’re seeing a nudge for them,” he says.

An ADU is a smaller, independent dwelling located on the same lot as another residence, often used as a guest house or rental unit, and something that changes the amount of wastewater flow on a property. Accounting for these trending additions is now standard procedure for Purdum, whether doing repairs or designing from scratch.

“The biggest thing is education to the homeowners because a lot of people think they’ll just put an ADU on their property and hook it into their existing septic system and everything is fine,” Purdum says. “And that’s just not the case. My job is to educate and guide clients through the whole process.”

When working with clients that have bare ground and building new, he will always ask about future plans and if they considered adding an ADU sometime down the road.

“Three-bedroom homes are very popular around here, and ADUs by most county definitions need two-bedrooms worth of flow assigned to them,” he says. “So, now we need a five-bedroom system on the property and a lot of people don’t realize that.”

Other issues may arise if they are on public water like a shared well or municipal water system. Many counties want those to have two separate water meters, one for the primary residence and one for the ADU.

“A lot of people will convert a garage or storage building to living quarters and bootleg it into their septic system,” Purdum says. “Then it comes time to sell the property and we have this illegal ADU that we now have to try and justify. It creates a mess.”

Despite the chaos they can create, Purdum welcomes ADUs with open arms. “From my business perspective, it’s a great source of revenue,” Purdum says. “All these people with established homes want to add ADUs and that creates a whole new stream of income and work for me.”

If people approach him with a three-bedroom house design, he’ll often discuss the advantages of planning for a larger footprint with the client.

“I let them know if they put a five-bedroom septic in, the cost goes up a little bit, but now you’re ready for a future ADU,” he says. “So, if they decide they want one someday, all they have to do is build it. I also make it known that the resale value increases having that added capacity.”


Purdum relies heavily on a Topcon Total Station for his site work along with a few vital tools and equipment.

“Of course, I can’t get by without my trusty dig bar and post-hole digger,” Purdum says. He also relies on a 2023 Kubota KX018 compact excavator. “It’s a great little machine,” he says. “For 95% of the jobs I work on it’s adequate. One of the biggest advantages is that the tracks can retract a little bit, so I can actually fit it through a standard backyard gate for repair work or anything like that on the tight lots.”

Another advantage for Purdum is the fact he can pull the smaller machine with his half-ton Toyota Tundra.

“Before buying this, I was using other installers to dig my test holes,” he says. “But the problem there is scheduling.”

He still gets a few jobs that require calling in someone with a bigger machine, which Purdum has no problem doing as he says it’s a great way to form relationships with installers that come in handy down the road.

Installer relationships are mutually beneficial. Purdum uses those who have proven themselves as top-notch workers and does what he can to get these proven installers on his projects. He says it goes the other way around too, and installers that he frequently recommends do the same for him. “If they are good installers and they are sending me work, I’m going to send it right back to them.”


Between the influx of ADUs, repair work and general new-system work, American Septic Design keeps a full plate. The majority of his work currently is made up of repairs and remodels, but he says new construction is picking up.

“New construction is a fun challenge,” Purdum says. “You get to go out on a property and beat your way through the trees with an excavator to dig test holes and that’s just plain fun. And repairs are like a puzzle. I just really enjoy both sides.”

Currently, Purdum is taking on as many jobs as he wants to and there’s more to be had in the area. “There’s plenty of work for everybody,” Purdum says. “I don’t see other designers as competition. We are all in this community together.”

Purdum says he’s not afraid to call other designers to ask questions and learn from them, and he’s had the same happen to him. “We try to elevate the design community and work together as much as possible,” he says. “Like they say, you’re never too old to learn something new.”

To Purdum, success in septic design is about talking and execution. “Take the time to listen to the client’s needs and follow through on what you are promising,” he says. “Delivering designs in a timely manner and helping the process, whether it’s new construction or a repair situation, go smoothly. Communication is everything.”


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