New horizons for sludge
Several drivers are contributing to the need for sludge solutions. A global population of 7.5 billion people, growing at a rate of 200,000 per day, is increasing production of sludge – and all other wastes – all around the world. Sludge problems are exacerbated by the increasing number of megacities, with their populations greater than 10 million.
In Europe, environmental restrictions and shrinking available space are making it more difficult to continue traditional approaches of disposal.
For example, Germany is taking steps to prohibit agricultural application of sludge and to require phosphorus recovery. Wastewater treatment plants in urbanized areas also face limits on truck traffic and all factors are contributing to rising disposal costs that can range from €20 to €150/ton.
In North America, attitudes are changing. Sludge is converted to biofertilizer that can be reused, including as a dry pelletized product applied in agricultural and landscaping.
Recovering nutrients such as phosphorus from treated sludge, called biosolids, is being driven by economics and regulatory pressures. Increasingly important is the recovery of water, which makes up to 99% of untreated sludge, especially in drought-stricken western regions.Attractive values
Appreciation also is rising of sludge’s energy-producing qualities as a means of fulfilling renewable energy goals. The European Union, for example, has set an objective of increasing to 20% the portion of energy generated from renewable sources by 2020 and to 27% by 2030. In the U.S., utility companies unable to build solar and wind farms fast enough to meet new renewable energy mandates are purchasing energy from sources powered by biosolids.
Proven technologies such as anaerobic digestion, thermal hydrolysis, co-digestion or thermal drying are enabling sludge to be converted into a valuable energy source, with uses varying according to country energy prices. In Germany, for example, sludge-producing wastewater treatment plants often consume the energy they produce. In France, plants such as the one being built in Cagnes, are able to export the green energy produced back to the grid while in the UK, sludge is collected from multiple sites and processed in big digesters, with the energy sold on the market.
Sludge is also increasingly being viewed and treated as a source of valuable by-products and agriculturally beneficial ingredients, such as phosphorus extracted from struvite. France-based Veolia Water Technologies has long regarded sludge not as a costly waste but rather a resource from which to extract value for the benefit of customers. The company is applying an array of technologies to produce energy and fertilizer products from sludge.Heated response
Coupling thermal hydrolysis with anaerobic digestion minimizes sludge volumes while maximizing biogas and green energy production. Thermal hydrolysis uses heat to break down sludge prior to treatment by anaerobic digestion, considerably increasing the biogas yield and reducing the quantity of sludge for final disposal.
One leading thermal hydrolysis technology, Bio Thelys™, is being applied to increasingly large projects. In Bonneuil, France, energy-efficiency is at the heart of a new wastewater treatment plant which will feature Bio Thelys™. Energy produced from wastewater will be used to heat the plant and surrounding buildings, while biogas produced from the sludge will be treated and injected into the grid. Dehydrated, digested sludge will be transformed into compost and spread in fields to facilitate agriculture.
In Denmark, the Billund BioRefinery, which features the Exelys® thermal hydrolysis process, treats wastewater and organic waste from households as well as from industrial sources to generate biogas for energy production, reducing the plant’s power requirement and creating additional income through the sale of surplus energy, both heath and electricity, to the local grid.
Both Bio Thelys™ and Exelys® technologies can be applied for thermal hydrolysis, depending on the clients need and existing conditions at site.Thermal dried pellets
Applying the Biocon™ thermal drying process allowed the municipality of Buffalo, Minnesota, to do much more than simply reducing the biosolids mass from its wastewater treatment plant, by 95%; it also resulted in 70-80% savings of the plant’s thermal energy requirements, helping reduce operating costs by 50% (compared to disposal of wet sludge). In addition to generating inexpensive renewable energy, the remaining biosolids are ready for land application.
The Biocon dryer is now also available at low temperature (70 to 90 °C), enabling the client to use different energy sources.Oxidation power
The Pyrofluid™ thermal treatment solution oxidizes organic matter contained within sewage sludge in a number of countries. At Marne-Aval in France, for example, steam from two Pyrofluid furnaces feed an electricity-generating turbine, contributing to meeting the plant’s energy needs.
In Hong Kong, Veolia designed and operates the world's largest sludge incineration facilities. With state-of- the-art incineration and flue gas treatment technology, the facility generates 14 MW of electricity that is used to power the plant while up to 2 MW of surplus electricity are exported to the power grid.Multiplying solutions
Another innovative solution is SOLIA™ Mix, a new generation of solar sludge drying. This process can achieve dry solids content up to 90%, reducing sludge volume and removal costs and opening multiple disposal routes, including agricultural reuse, composting, landfill, incineration and co-incineration. Among Veolia’s references is Belchatow, Poland.
Veolia’s sludge expertise doesn’t stop with energy recovery solutions. The company is constantly looking for new ways to derive value and produce materials that are recyclable or reusable. Its Ecrusor™ Depackaging solution turns food and other waste high in caloric value into useful energy by depacking the waste from its packaging in order to extract the organic material. The process creates a homogenous mixture of liquid and solid organics that can then be conveyed to an anaerobic digester to produce biogas, which can then be converted into energy for use at the facility or for sale back to the grid.
As the multiple drivers combine to increase the demand for new sludge management solutions, Veolia is continuing to work on new technologies and upstream improvements. These include advanced instrumentation and control technologies to optimize the environmental and energy performance of facilities. Work is ongoing as well to improve existing approaches such as the introduction of low-temperature dryers capable of being used with a heat pump.
Finally, Veolia’s patented TurboMix™ mixing system provides improved phosphorus recovery capabilities through its compact, resource-efficient Struvia™ process. In the U.S., new regulations driven by eutrophication concerns and recognition of the potential value of the phosphorus is making recovery strategies increasingly popular. As a result, Veolia is working to increase the Struvia™ system’s application range in other parts of wastewater treatment and industrial phosphorus recovery.
Clearly, the future of sludge has never burned so bright!