In 1983, Minnesota paved two airport roads with asphalt mixed with sewage-sludge ash; by 1990, it was on three highways, and a federal review found no visible difference

A unique experiment in constructing roads in Minnesota illustrated how the material that was left over from the sewage treatment process could be recycled as well as used for building purposes. The engineers did not want to discard all of the ash from sewage-sludge incineration, but instead wanted to use it in asphalt road construction. The utilisation of this material would help solve two problems: the growing quantity of waste from the sewage treatment plants and the utilisation of materials in road construction.According to the Federal Highway Administration (FHWA), Minnesota started incorporating ash from sewage sludge into asphalt in 1983, when two bypass roads at the Minneapolis–St. Paul International Airport were constructed using asphalt that contained 2% or 3% ash. Additional demonstration projects were undertaken in 1989 and 1990 and included the construction of the I-94 bridge overpass as well as segments of TH–47 in Ramsay and TH-97 in Forest Lake Township.Turning sewage waste into rad building materialSewage sludge is the residue left after solids are separated from wastewater during treatment. Incineration of the sewage sludge leads to the formation of ash residue, which needs proper handling. The Metropolitan Base Control Commission of Minnesota, currently operating under the Environmental Services of Metropolitan Council, has initiated a study on the use of this residue in construction rather than disposal. The study was focused on the use of sewage sludge ash as mineral fillers in hot mix asphalt (HMA). Mineral fillers are useful for occupying spaces between aggregate particles in HMA. Therefore, the finely divided nature of the ash made it a suitable replacement for materials like stone dust and hydrated lime.Initial investigation by the Minnesota Department of Transportation examined the physical properties of the ash and its behaviour in asphalt mixtures. Research published in Construction and Building Materials has also studied the use of sewage sludge ash as a substitute for regular mineral failure in asphalt pavement mixtures, considering aspects like stability, density, and air void content. The Minnesota experiment was especially noteworthy since no special road construction methods had to be developed. The ash could be incorporated into regular asphalt mixers without any need for a new method of mixing, laying, or compacting the material.From airport bypass roads to state highwaysField demonstrations were conducted in 1983 at Minnesota Saint Paul International Airport. Two bypass roads utilised sludge ash as a constituent of pavement layers such as base binder and wearing courses. These mixtures used either 2% or 3% ash, depending on the test, and the I–94 bridge overpass project used a wearing course in 1989 and 1992. A segment of TH-47 in Ramsay was made with a 2% ash content in the levelling and wearing courses, while TH-97 in Forest Lake Township was constructed with a 2% ash content in its base, binder, and wearing courses. These projects provide possibilities for testing this material in real traffic and weather conditions, unlike a laboratory setting. FHWA reported that there was no apparent difference between sections with sewage sludge ash and conventional sections. It is quite promising, but this does not mean that these pavements perform equally well across all settings.What the federal review revealedThe FHWA Technical Assessment included more detail on how sewage sludge ash can be used in asphalt mixtures. It is pointed out that the properties of a mixture that includes 2 to 5% ash, in relation to the weight of aggregate, can be equal to those of mineral filler. It was suggested that the amount of ash should be limited to 5% in relation to the aggregate weight and 3% in case of wearing course. The incinerated ash might also have some environmental impacts, but considering this, the team has suggested using a minimal amount.However, the organisation indicated that, despite the successes of sewage sludge, it has not yet been accepted for broader commercial use in the USA. Such implementation requires specific material requirements, mix design standards, and environmental monitoring. Nevertheless, an experiment conducted by the state of Minnesota provided an available example of recycling waste in transport infrastructure.You use AI every day. Now get your AI Quotient. Take the AIQ test.