Digging deep for a waste solution
Ryan O’Loan from CDEnviro looks at the how the resources in excavated material can be maximised, with pollutants removed and disposal costs reduced.
We live increasingly urbanised lives, with a resulting pressure on space and infrastructure. Add to this our growing technological demands, requiring miles of cabling, and ‘digging up a road’ or around a building’s foundations has never been so difficult. That’s why hydro excavation – the non-mechanical/non-destructive process combing pressurised water with a high flow of moving air to simultaneously excavate and evacuate soil in a controlled manner – is so important. It is much more effective and generally safer than more traditional methods of excavation.
The hydro excavation technique, used in urban excavation, means it is possible to work safely around underground utilities, including gas, electricity, telecommunication services and water and sewer pipes. The high-pressured water moves the surrounding soil without the risk of pipe ruptures. The resulting waste soil and water slurry are then transferred by vacuum pipes to be disposed of; usually in a truck-mounted debris tank.
However, while it offers many advantages, the end result is that there is still a lot of waste material to deal with and it’s very wet, increasing the weight and making disposal a costly problem.
From the depths
The composition of hydro excavation (or ‘hydro vac’) waste varies depending on the type and location of the site being excavated. Generally, it comprises sand-fine aggregates, organic matter and an elevated level of fine silts and clays.
This type of waste – including hydro excavation muds, dredged materials, drilling muds or other thixotropic materials – can be difficult to dispose of due to its solid/liquid ‘semi-state’, the solid contents (which often contain both contaminants and reusable resources) and the viscosity of the liquid.
Finding a disposal site for this solid/liquid waste is difficult, and even where it is accepted, it is not a cheap solution due to its nature and weight. As well as that there are often contaminants in the soil and treatment water which should be removed before disposal is considered. There are other reasons why this method of disposal shouldn’t be top of the list.
The material contains a lot of useful, reusable materials that can be recycled and made available for resale, turning a loss into a potential profit. Treating the material in this way is also much more in line with circular economy philosophy which aims to keep resources ‘in the loop’ for as long as possible. This contrasts with the traditional linear approach of extract, use and dispose.
A further constraint on just disposing of the excavated material is that, as legislation becomes more stringent, the payload per truck will be reduced, and there is an increasing need for urban recycling solutions.
What are the alternatives?
New tailored solutions can provide bespoke reception centres for receiving hydro vac waste; processing it to recover sand, stone and organics through the system to produce independent revenue streams, and dewatering the final clay content to ensure easy and cheap disposal.
The recovered water can then be reused to fill out-going trucks with industry compliant recycled water. Beyond this, if heavily contaminated, this hydro vac waste can be further processed to remove heavy metals and hydrocarbons from the wastewater stream. This means it’s now possible to process a wider range of more difficult (and therefore more lucrative) waste streams.
These types of solution provide our industry with greater control of our waste; greater sustainability, reduced disposal costs, a secure method of processing, and reduced business time and money spent on transportation.
Due to the varying nature of the materials in need of treatment, there’s no one solution that will always be appropriate. However, several systems are now available that can be combined to take on any challenge.
These include systems that screen and scrub material to ensure effective removal of contaminants. This involves a density separation process, attrition and high-pressure washing. The result is clean and separated stone, sand and organic material, all of which can be reused.
Fine particles which would otherwise settle at the bottom of debris tanks can also be an issue, reducing capacity and causing wear and tear on pipe works. There are systems to remove this debris, helping to minimise the volume of waste to be processed downstream.
In some situations, a multi-stage chemical dosing and application process – with the ability to treat ultrafines and other elements that would typically cause an issue in the settlement process – is required. These systems offer very high levels of water quality with ultra-low residual solids, while enhancing the performance of the settlement process.
Finally, using centrifuge technology, solids and liquids can be separated in a high-speed process maximising resource recovery and the amount of recycled water.
Hydro excavation is a safe, non-destructive process and its by-products can be so much more than problem waste. With the right systems, resources can be extracted from the most unlikely sources creating economic and environmental benefits; something it’s worth digging deep for.