What is in my “Dirty Pervious Pavement?”

The fact is that the percolation rates of pervious surfaces will decrease over time and finally clog if not properly tended to.  So, what will we 1) pick up when we clean it? And 2) How do we dispose of what we pick up?  

What Makes Pervious Pavement So Dirty?

The top 3/8″ to 1/2″ of pervious surfaces contain lodged organic and inorganic materials. High volumes of fine solids will be suspended in the water used to dislodge the material, which will then be transported back to the recovery tank.  This will include organic and inorganic material. A lot of fine solids will be suspended in the water used to dislodge the material and transport it back to the recovery tank.

The recovered material will include anything and everything that has settled on top of your hard surface, too.  This includes but is not limited to organics, phosphates, heavy metals, oil and gas and tire particulate from car traffic.

What Do I Do With That Stuff?   

Plan to treat the recovered material like you do the debris recovered from your catch basins. 

dirty pervious pavement

Pervious Cleaning at an Amtrak facility. Anything and everything has accumulated in and on this pervious surface.


dirty pervious pavement

Close up view of the Amtrak facility’s pervious asphalt. The pervious surface clogging goo seen on the left (not yet pressure washed) is pollutant rich. Get it out of your storm stream and into your treatment stream.


dirty pervious pavement

Contain and transport the slurry generated by the cleaning process. Best overall results are achieved by means of a high-pressure wash with simultaneous vacuum recovery system as found on the Triverus MCV. Full of fines and pollutants, this material should be hauled to your decant facility for treatment.

What’s the Difference Between Dirt on Pervious Surfaces and Non-Pervious Surfaces?

It stands to reason that both your pervious pavement and your non-pervious pavement will accumulate fine solids and pollutants from ambient sources.  The only difference is that when it rains, the pollutants and solids on a non-pervious surface are not trapped in place like the material that gathers sub-surface in pervious pavements.  

A high-pressure wash system with simultaneous recovery will lift and contain solids and pollutants from any hard surface. A conventional street sweeper only applies enough energy to the surface to move superficial, loose material. Adhered pollutants and those that have moved subsurface into the pervious pavement are beyond the job description of a conventional street sweeper regardless of vacuum recovery or regenerative air. Once wet, fine solids and pollutants will adhere to the surface and can only be moved by a soaking rain or high-pressure water wash.dirty pervious pavement

dirty pervious pavement

High-pressure washing with simultaneous recovery of both pervious and non-pervious surfaces yields the added benefits of enhanced safety as the pavement markings become more visible

Pollutant Recovery from Pervious Pavement and Other Surfaces

For a closer look at what you are pulling from your pervious and non-pervious surfaces, in 2019, the University of New Hampshire Stormwater Center analyzed the material recovered from two pervious surfaces during percolation recovery operations.  The chart below summarizes the analysis of the material recovered in terms of pounds per acre. (note the logarithmic scale). The complete study can be accessed here: University of New Hampshire Stormwater Center Tests Pervious Pavement with Triverus at Durham, NH Campus

dirty pervious pavement

Does the Type of Dirt on a Road Vary By Location?

Depending on your location and application, the mix of material and pollutants recovered will vary.  Cleaning western roadways will yield considerable amounts of copper while the same activity in the East tend not to return as much copper.  Industrial sites or roadways near them will be loaded with more heavy metals contingent on the local industry involved. 

Research has also shown that the tire preservative 6PPD is ubiquitous and deposited on roadways during the tire wear process.  6PPD is setting off alarm bells in the Pacific Northwest with studies linking it to high mortality rates in Coho Salmon.

High-Pressure wash with simultaneous recovery can be a key tool in removing all these pollutants from any hard surface.  Using this cleaning method on pervious pavements has the added benefit of retaining and restoring percolation rates to keep stormwater outflows to a minimum.  

Jumpstart solutions to your pervious maintenance and general pollution problems with Triverus. Get in touch with us today.

How to Prevent Flooding of Pervious Surfaces

“The best tool you can use to verify your pervious pavement is functional . . . . is an umbrella!”  Ever heard that one?  How about, “Why don’t you fix your leaky roof?”  “I have wanted to fix it when the weather is nice, but when the weather is nice, it is not leaking.”  It is the same principle.


Basketball Courts in the Pacific Northwest

“It was pervious when we installed it, anyway.” Although this is an extreme example, this can be a temporary condition while it is raining and for a few hours after. Use an umbrella to inspect pervious pavement.

What Causes Pervious Surfaces to Flood?

Pervious pavement does not clog overnight.  Plugging accelerates during and just after periods of high biological loading.  Although conditions vary across the United States in terms of rainfall, tree cover, and growing season, the general rule applies that spring and fall are the heaviest biological loading periods of the year.

Added to this is the fact that pervious pavement is constantly subject to infiltration by fine solids, dust, and dirt all year long from local and ambient sources.  We can talk about the well-meaning gardener that dumps his mulch and/or topsoil directly on your brand-new pervious surface later.

In the case of the recreational basketball courts pictured above, the percolation rate has deteriorated so badly it was almost nil when this picture was taken.  The inspection and discovery time for a pervious surface in this poor condition is not time-sensitive and can be seen for days after a rain event.

You will probably hear about it from local future NBA stars as well. The environmental benefit from this particular investment has not been realized for many years.  Inspection by umbrella would have helped identify the deteriorating pervious condition and encouraged early and routine maintenance and consistent functioning of this installation.

Materials that Clog Pervious are Transported by Rainfall


Northeastern United States residential street second half of October littered with leaves. Traffic will pulverize the leaves. Crowned conventional pavement delivers rainwater and pulverized leaves to the precast pervious concrete blocks in each gutter. It only took 16 months for this pervious concrete to clog.

From the west coast basketball court, let’s go to an East Coast residential street.  Because gravity is a thing on both coasts, water from grassy areas above the basketball court pictured above adds to the drainage and contaminant load for the pervious pavement.  In the typical residential street pictured, leaf litter is pulverized by car traffic across the entire crowned conventional asphalt surface.  Local residents knew it was failing because they saw the puddles during and directly after each rainstorm.

Pervious Clogs Because it is Doing Its Job

Rainfall effectively delivers the pulverized organic matter and airborne inorganic dust and dirt to the precast porous concrete blocks from the entire width of the street.  Because pervious asphalt and porous concrete are both constructed with voids to let the rainwater pass, gravity deposits these solids in those voids.  This natural process gradually plugs the voids.  Conveniently for us, the bulk of such material is filtered from the water and collected in approximately the top 3/8” of the pervious surface.



Pervious Asphalt Parking Lot in full failure mode. It is raining in this picture, too, but an inspection much later in the day would still show this pervious pavement’s condition.



Same parking lot same day: small demonstration area made functional again with high pressure water and simultaneous vacuum recovery – MCV



Rainwater running down a pervious asphalt street. Two ways to discover this: 1. Drive or walk by with your umbrella while it is raining; 2. Your crew does an ASTM perc test, disrupting traffic, any day of the week.

How To Maintain Your Pervious Surfaces 

  1. Regularly sweep your pervious surfaces with conventional sweeping equipment.  Keep as much organic and inorganic litter off the surfaces as possible.
  2. Inspect your pervious surfaces using an umbrella during rainstorms to note percolation deterioration early.
    1. To learn more and as needed, document the deterioration by means of the conventional ASTM test procedure
  3. Conduct pervious pavement maintenance cleaning regularly as indicated by the results of your observations and tests described in #2. Subsurface pervious cleaning requires a high-pressure wash with simultaneous vacuum recovery (MCV).
  4. Pervious pavement functionality can be restored at sites like the flooded areas shown in this posting.  The recovery or restoration process requires a more intense treatment with the same high-pressure water and simultaneous vacuum recovery that is used for periodic (seasonal or annual) maintenance.


Morals of the story

Don’t pretend your pervious surfaces will not clog over time. Don’t write off your investments in pervious pavements without a fight. See it to believe it.  Get in touch with Triverus for a demonstration on your site.

Pervious Pavement and How to Clean It

When Pervious Pavement Penetrated the Market

In the early days of pervious installations, the importance of maintenance was rarely mentioned.  Back in the day, I listened to a host of experts on the subject showing their pervious installations and forgetting the important discussion of maintenance once installed. The Pacific Northwest was especially aggressive with the new technology because their interest was focused on protecting Puget Sound in a very rainy environment.  

Even there, rarely did you hear the word maintenance in APWA and other stormwater conferences.  A hard, paved surface was treated as a hard, paved surface.  Not surprisingly, BMP for hard surfaces were generally not changed.  The cleaning process for pervious pavement practice for years had been and continued to be conventional mechanical street sweeping, or brush type sweeping enhanced with vacuum or regenerative air retrieval systems.  

Improved Pervious Percolation Performance Through Appropriate Maintenance.

The unique conditions in the Pacific Northwest, however, lent itself to a tattle tale visual cue that new techniques were needed to maintain the new pervious surfaces to best effect.  Accumulated dirt and organic material filtered out by the pervious surface provides an excellent growing medium for moss.  

In the Northwest, when pervious sidewalks and parking lots became green and slick when wet, it was the pervious installations crying out for help.  Dirt and organics filtered out by pervious in other parts of the country just do not have the climatic advantage of that in the Northwest, so the pervious surface cannot so loudly cry out for help – and improved maintenance practices

Subtle and Not-So-Subtle Views of Clogged Pervious

cleaning pervious pavement

Street View. Note lack of surface texture in upper left area of photo. “The filter was full.” Lower right shows the pervious after one pass of MCV emptying the filter


cleaning pervious pavement

Macro parking lot view. Can’t miss this cry for help in the Pacific Northwest. (Im)permeable concrete being made permeable again. Path made by single pass of MCV.


While both the areas pictured had been swept traditionally on a regular basis, neither of them had any meaningful percolation rate prior to proper treatment to bring back the original functionality.

Keeping Pervious Effective Cost Effectively

It is a FACT that sooner or later your pervious pavement will clog and be less effective than when it was new. All sites have their own characteristics.  Identify the sources of the debris and dirt that are migrating into your pervious:

  • Trees and nearby or overhanging vegetation;
    • Particularly seasonally in spring (budding) and fall (shedding).
  • Run-on from nearby unpaved areas, especially nearby bare slopes
  • Road traffic 
    • This is also a large source of hazardous pollutants that will transport via stormwater.
  • Road sand / cinders. (!)
  • Piling plowed snow, with dirt and debris with it, on a pervious surface
  • Designed inflows from larger areas to the pervious receiving area
  • The landscaper that dumps his truck load of topsoil or mulch directly on the pervious for his momentary convenience.  (!)  — you know I am not kidding, right?
  • Airborne dust and particulates
  • (Coastal) Bird excrement. (!)
cleaning pervious pavement

Totally covered and clogged pervious sidewalk downhill from, and adjacent to, an untended bare slope. Photo from fall, 2020 during percolation recovery operation. Yes, it was brought back to a useful life.


The fact is that while effective recovery and periodic maintenance require high pressure wash applied with simultaneous vacuum, conventional sweeping has a role to play in extending the periods between deep, sub-surface cleaning with high pressure water and simultaneous vacuum recovery.  

The most cost effective, long-term maintenance for your pervious surfaces will consist of a judicious, well-thought-out program using both methods.  A deeper dive into balancing these two cleaning methods is provided in Triverus’ Pavement Maintenance Cleaning Guide.

Through contract service work for municipalities that are still expanding their use of pervious pavement, Triverus has experience with a large number of variable pervious pavement applications and conditions. 90% of our contract service work has been aimed at recovering percolation rates of clogged pervious asphalt and pervious concrete, while only about 10% could be categorized as maintenance of pervious pavement

Municipal satisfaction with percolation recovery results is now leading more and more often to contracts for pervious maintenance work or vehicle procurement by the municipalities themselves to establish consistent pervious pavement maintenance programs.  This trend is enhanced where state or Federal assistance is available (e.g. Washington State) to assist local procurement of pressure wash /simultaneous vacuum type equipment. 

Want to talk more about methodology through email or a site visit by Triverus? Get in touch with us today.

Should Pervious Surfaces be Part of My Stormwater Plan?

The simple, but unhelpful, answer to this question is, “maybe.” There are multiple factors that can influence how pervious surfaces handle stormwater. Among criteria to consider are the soil beneath, the flow of water, and the traffic the surface experiences.

What type of soil substrate do you have?

The first criteria are the characteristics of the soil at your intended installation point. Installing a pervious surface above an impermeable substrate will not work. However, if your substrate is penetrable, then you can take advantage of that fact by allowing stormwater to reach the substrate.

As an example, a major East Coast City selected a sub-division to test the effectiveness of pervious surfaces in controlling stormwater outflows.

  • First, they spent over a year collecting data on the amount of stormwater that was flowing out of the subdivision and entering the neighboring environmentally challenged bay
  • Second, because the residential streets are crowned and relatively narrow, the city engineers opted for 4-ft wide precast pervious concrete blocks lining the edges of the streets.
  • Third, after the installation of the pervious blocks, they continued to collect data on stormwater outflows from the area.

The data showed that stormwater outflows from the study area were reduced by 60%.

Based on these excellent results, the city is continuing to expand its installed base of pervious surfaces in other areas of the city.

stormwater pervious surface

Precast pervious concrete blocks installed on crowned street in East Coast city. The crowned conventional impervious roadway delivers the stormwater to the pervious surface for processing.

Stormwater and Solids

The stormwater issue is not just about the water itself, but what stormwater run-off carries with it.  You don’t really see it because we are so used to it, but traffic bearing paved surfaces are filthy. Paved surfaces have a coating of tire residue (including copper and the tire preservative 6PPD that has been proven to decimate salmon in the Northwest), hydrocarbons from our cars, pulverized organic matter, dirt (both fine and coarse), and so on.

Just look at your local white shiny crosswalks if you want to see the amount of dirt and residue that covers our city streets.  A lot of this stuff is adhered to the surface of the pavement.  It is stuck.  It tends to become unstuck when it rains.

triverus pervious surface

Typical crosswalk, except the brighter black and white area in the rear has been pressure washed to remove adhered solids.

How Does Traditional Street Cleaning Work?

Think of the stuck cheese and dried marinara sauce on your pizza plates when you save the clean up for the morning after your party. How do you make cleaning up easier? You soak the plates in water.

Streets go through the same process.  Paved surfaces become a storage area for adhered solids, heavy metals and pollutants.  The conventional street sweeper, with or without vacuum or regenerative air, moves anything that is loose, including dixie cups, cigarette butts, and dust.

Much of that dust escapes into the air only to settle on nearby cars and back onto the just-swept street.  All those adhered solids are still there when it rains the next time. The rainwater loosens many of these solids, and into the storm drain and out to the river or bay they go. This is not a good time for the EPA to show up at your outflow with its sampling equipment, test kit and a copy of your stormwater plan and commitments.

How Do Pervious Surfaces Work With Stormwater?

If we catch and redirect the rainwater from conventional hard surfaces into and through the pervious surface, we will also accumulate the solids and pollutants without letting them flow to the storm sewer and on to the river or bay.  A pervious installation is a giant water filter, letting the water flow back to the aquifers below the surface while the pollutants are collected in the upper levels of the voids of the pavement for cleanup and disposal.

Can a Traditional Street Sweeper Clean Pervious Surfaces?

A conventional street sweeper with or without vacuum does not exert enough energy on the paved surface to break the adhered material free, nor can it apply effective amounts of energy into the pavement voids where the rain loosened solids have collected for us.  Conventional sweeping will extend the original functionality of a pervious surface, but inevitably, enough solids collect to reduce and eventually eliminate stormwater percolation.  A hygienically applied high pressure spray and recovery is required to finish the job and to keep pervious surfaces functioning.

Contact Triverus to set up a demonstration of how Triverus technology can recover your pervious flow rates at your location. 

Clean Landing — SBIR- STTR Success Story

SBIR- STTR Success Story for Runway Cleaning

A Mobile Cleaning, Recovery, and Recycling System Helps with Runway Cleaning and Gets Naval Aircraft Carriers and Municipal Parking Lots into Shipshape. 

A Navy aircraft carrier idles in the middle of the ocean, readiness compromised by foreign object debris (FOD) across its flight deck. Debris and hard particles have built up from aircraft and ground-support activity. Even something as routine as moving planes from the hangar deck to the flight deck can spread debris or spill grease and jet fuel. Given that jet engines have powerful intakes that can suck up debris, the risk of damage to aircraft engines becomes too significant to allow takeoffs and landings. Even a bit of spilled grease or fuel can be dangerous if aircraft slip into each other on the rolling deck of a carrier on high seas.

In order to get the carrier fully operational, the Navy calls on Triverus, a Palmer, Alaska-based small business that has produced advanced cleaning technology since 2001. Triverus ships their five-ton Mobile Cleaning Reclaim Recycle System (MCRRS, pronounced “McChris”) to the vessel overnight. MCRRS immediately starts runway cleaning on the deck using water-jet technology, integrated air recovery, and waste-water recycling. Within a short time, the flight deck is spotless, and the ship certified as mission ready.

Read the full article


University of New Hampshire Stormwater Center Tests Pervious Pavement with Triverus at Durham, NH Campus

UNH Tests Pervious Pavement Functionality with Triverus        UNH Tests Pervious Pavement Functionality with Triverus

Before Triverus MCV Cleaning — Water Running Off       After Triverus MCV Cleaning — Water infiltrating


In the summer of 2019, the University of New Hampshire Stormwater Center invited Triverus to their campus in Durham, NH, to determine the effectiveness of the Triverus Municipal Cleaning Vehicle (MCV) in pervious pavement cleaning and restoring percolation rates. UNH has several generations of pervious pavements, each installed when major changes or advancements in pavement design or installation occurred.  While each plot is still in use, predictably, percolation rates deteriorated over time.  By the summer of 2019, all their pervious paved surfaces were non-functional.  Over the period of a week, the MCV performed trial pervious pavement cleanings on five such plots. Every treated test plot showed restoration of acceptable percolation rates, even after up to 15 years of non-functionality.  UNHSC published a full review of the process and results of testing in the parking lot of their Alumni Center. Read the study here.

Palmer Company’s Cleaning Machine Deployed to Navy Flightdecks

Palmer Company’s Porous Cleaning Services - Navy Flight Deck

PALMER, Alaska (KTUU) — Technology pioneered in an Anchorage garage and produced in Palmer is making its way onto Navy aircraft carriers.

Since the early 2000s, Triverus has been working on a porous pavement maintenance machine that efficiently cleans flight decks and removes everything from grease and oil to debris.

“That can get sucked into jet engines and destroy planes awfully quickly, so what the Navy does right now is what’s called a FOD walk where they just send sailors out to one end of the carrier and they literally walk multiple times a day and just stare at the flight deck and pick things up,” Triverus manufacturing manager Nathan Green said.

The current methods used to clean flight decks inhibit the Navy’s ability to respond immediately when aircraft need to be deployed.

Triverus won a contract from the Navy to build 43 mobile recycling cleaning system MCRRS vehicles. Each machine costs $850,000.

It works by using pressurized water, magnets and a vacuum to remove debris from the flight deck. The debris is then filtered out and the water recycled.”

Click here for the full KTUU article

Palmer Business Sends First Military-grade Machine Overseas

“A Palmer manufacturing company is sending its first piece of military-grade machinery overseas.

The Triverus product is called MCRRS, short for Mobile Cleaning Recovery Recycle System. It’s basically a high-tech water vacuum designed to clean debris from flight decks on Navy aircraft carriers.

Each machine takes about one month to make and costs around $850,000.

The company has made a handful for the U.S. military. Now it’s filling its first order for the Italian navy.

CEO Hans Vogel said Triverus shows large-scale manufacturing is possible in Alaska.”

Learn more about the MCRRS and our other vehicles. 


Copyright 2019 KTVA. All rights reserved.

Triverus MCV Case Study: Minneapolis-St. Paul International Airport

With its large 60 inch deck and high-efficiency technology, the Triverus MCV Vehicle is unmatched in achieving regulatory compliance through the removal or containment of restricted materials. The Minneapolis-St. Paul International Airport was able to save over $4 million, have 111% faster results than manual cleaning, and retained 95% water recovery and reuse. In addition, the new MCV vehicles allowed crews to clean the same 500-space area in two days, rather than seven.  Click here to read more!

Triverus MCV Vehicle Case Study: MSP Airport

“Palmer Company’s Navy Contract Highlights Alaska’s Export Market”

“While some may say there are no large manufacturing export markets in Alaska, one Palmer business is proving otherwise.”

Click here to read more in the Mat-Su Valley Frontiersman!

Photo Credit: Mat-Su Valley Frontiersman