Sludge: Turning waste into a valuable resource
A range of processes are required to remove potential health risks, remove water and reduce the volume and weight of sludge to improve the costs and options for disposal. With the correct treatment, sewage sludge can be transformed from what was considered a waste into a valuable resource and sellable product, also known as biosolids or bioresources.
Sludge contains the organic matter that is separated from and created during the treatment of wastewater. Biosolids are produced once this sludge has undergone treatment to remove pathogens and create a stable product.
Biosolids can be used as an alternative to manufactured fertilisers to help play a role in soil quality and fertility improvements, as well as stimulate plant growth. As well as the potential production of an agricultural fertiliser, industrial and municipal sludge can be used to produce a biogas for power generation.
Depending on the country, biosolids must meet strict regulation and quality standards before being applied to land. The treatment and land application practices of biosolids and sewage sludge vary considerably from country to country. In some regions, sewage sludge undergoes advanced treatment and land application is common, whereas in other regions such treatment is unusual and sewage sludge is disposed as waste.
Most countries where land application is practiced have in place regulations or guidelines defining the quality levels to be fulfilled for a safe application. These criteria are typically based on heavy metals and pathogen contents and apply to the input materials and/or the final product.
What is sewage sludge?
Sludge is defined as the solid, or semi-solid and liquid residue separated from or created during municipal. Wastewater treatment plants may also receive imports of sludge from other facilities, such as household septic tanks or small treatment plants. Due to the enormous quantities of wastewater being treated around the world, this organic waste must be properly managed.
There are two main types of sludge: primary and secondary. Primary sludge is principally the settled solids removed from the wastewater through sedimentation. Secondary sludge is principally the biomass created through biological treatment, including activated sludge and trickling filters.
Sludge can be rich in nutrients, including nitrogen and phosphorus. As a result, once treated, this organic matter can be spread onto land as a fertiliser or an organic soil improver. However, depending on the inputs to the wastewater treatment plant, and the processes used on site, the sludge can also contain heavy metals and poorly degradable trace organic compounds. Monitoring for these substances and applying a risk based approach to the use of biosolids to land can help mitigate any problems. Such an approach is set out in Directive 86/278/EEC, and applied across the EU, as one example. Switzerland, Germany and The Netherlands have restricted to recycling of sewage sludge to land due to concerns of its risk; instead the incineration of sludge is common practice.
Wastewater sludge process
The treatment of sewage sludge for reuse is known as sludge processing. The aim of sludge processing is to reduce the volume of sludge, and where its recycling to land is considered, to reduce the amount of pathogens and create a stable product.
A side aim is to generate biogas for energy production, and / or to recover useful products such as struvite (phosphorus). There are several ways of treating sludge for these purposes, but they all generally involve thickening, dewatering and a step such as anaerobic digestion, composting or thermal treatment.
The sewage sludge treatment process can include the three stages of thickening, digestion and dewatering processes, detailed below.
Thickening: this can take place prior to digestion and dewatering facilities, as well as following digestion and dewatering. According to Suez, thickening is essential to reduce the volume of the sludge generated by wastewater treatment. This optimises conditioning, stabilisation and dewatering by reducing the size of the facilities and equipment, and the operating costs. Thickening technology choice is usually determined by the size of the treatment plant and space availability. Commonly used sludge thickening processes include dissolved air flotation and rotary drum thickening, as well as centrifuge thickening, although this is not as common. Typically, thickening results in the range of 2-8 per cent dry solids (ds) and after dewatering generally in the 20-35 per cent ds range.
Dewatering: ahead of the final disposal, remaining sludge is dewatered. With the sludge still containing high levels of water, it’s very important to dry and dewater the sludge. Various processes can be used for this process: sludge-drying beds are a common method, although can be time consuming. An alternative are solid-liquid separate devices, including centrifuges, rotary drum vacuum filters, belt filter presses, screw-press and centrifuges. For the centrifuge process, it enables the water to be retrieved and enables the easier handling of solid waste in shorter durations, at reduced costs, according to Water Online. Furthermore, to help improve dewatering flocculation aids can be added – polymers mainly based on poly-acrylamide.
Digesting: using anaerobic digestion (without oxygen), digesting sludge can reduce the material in volume by degrading the biomass and recover a biogas. This can be a useful application, with utilities facing a rising production of sludge and increasingly difficulty for disposal, such as costs and regulations. Anaerobic digestion uses anaerobic micro-organisms to convert organic matter into a gaseous product, including methane. There are two main options for digestion: mesophilic (36°C) and thermophilic (50 or 55°C), which can impact the biogas yield.
Sludge pumps: designed and built to be tough
Sludge is pumped between treatment processes, and can be conveyed in a more liquid form across a site.
Sludge pumping requires dry or submersible wastewater pumps that are specifically designed to pump liquids with a high solids content. In pumping the viscous liquids containing various amounts of solid particles, the pumps are designed so that they do not get clogged by the slurry and sludge content. It’s paramount that these pumps are not adversely affected, degraded or get clogged.
As they have to pump water containing items such as gravel and silt, many pumps are designed for heavy-duty centrifugal pump applications in which debris prevents the use of standard, centrifugal pumps.
Furthermore, a high powered mechanism is essential to pump sludge as not only can it contain corrosive and volatile content, but it is also heavy. According to Premier Fluid Systems, it’s important to choose the right pump for the right category: settling and non-settling.
For settled sludge, coarse particles form, leading to an unstable mixture. Because of the flow and power required with pumping, it is essential to select the appropriate pump. As for non-settling sludge, this consists of fine particles with low-wearing properties.
What are biosolids?
Biosolids is a term used, particularly in the United States for the product created following sewage treatment that can be used as a soil conditioner.
Biosolids can contain essential plant nutrients and organic matter that can be recycled as a fertiliser, as well as a soil additive.
According to the Environmental Protection Agency (EPA), biosolids are a “nutrient-rich organic materials resulting from the treatment of domestic sewage in a treatment facility. When treated and processed, these residuals can be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth”.
It's important to differentiate between biosolids and sludge, and this depends on the region. In the US, biosolids is the product generated following sewage treatment. However, sewage sludge is the solids collected from the wastewater treatment process that hasn’t undergone further treatment. Meanwhile, in Germany sludge is still applied to land.
The term biosolids was introduced in the early 1990s to help differentiate sludge that had been treated and deemed suitable for use on land.
Biosolids may contain macronutrients, including nitrogen, phosphorus, potassium and sulphur, according to the Australian Water Association. Micronutrients can include copper, zinc, calcium, magnesium, iron, boron and manganese may also be included in biosolids.
As it includes the major crop nutrients of nitrogen, phosphate and sulphur and others, biosolids can used as well as manufactured fertilisers, said UK water utility, Severn Trent Water.
Biosolids that meet strict criteria and application rates have been shown to “produce significant improvements in crop growth and yield” when used in the agriculture, according to the EPA.
Containing organic matter, biosolids can help to improve the “workability and draining of heavy soils”, the water utility said, adding that it can help to increase “the water-holding capacity of light soils, reducing drought risk in periods of prolonged dry weather”. It improves soil biological activity (such as bacteria, fungi, and the numbers of earthworms) and generally increases soil fertility all round.
There can be variability in biosolids quality, depending on the source of wastewater, the treatment steps and the dewatering processes.
Furthermore, the storage time ahead of land application can also make a big difference to nutrient concentration.
In the US, it’s estimated that around 50 per cent of all biosolids produced are used on less than one per cent of the nation’s agricultural land. In the UK, around 80 per cent of biosolids are recycled to land but this is linked to population density and the arable land available.
“As more wastewater treatment plants become capable of producing high quality biosolids, there is an even greater opportunity to make sure of this valuable resource”, the EPA said.
The future of biosolids
There has been some attention on sludge and biosolids and the impact they may be having on the environment and public health. As wastewater treatment plants are required to remove more and more substances such as metals and pharmaceuticals, inevitably some of these will end up in the sludge.
Within the UK there has been some initial research on the presence of microplastics within sludge; given the high removal rates seen through wastewater treatment plants it is not surprising that they are found in the sludge.
Further research will look to understand whether the levels found are significant or not, especially compared to the other routes to the environment of microplastics.
In some countries, the application of biosolids to land is restricted, primarily due to concerns over long terms environmental impacts regarding heavy metals and organic contaminants. However, this can also be due to public pressure and perception, rather than scientific data.
Within the US, there has been some attention of the human health impact of applying lower quality biosolids to land.
Towards the of 2019 the Guardian published an article headlined ‘Biosolids: mix human waste with toxic chemicals, then spread on crops’.
It referenced findings from the Sierra Club, as well as a study from the University of Georgia that linked an increased risk of illness to sewage sludge being used as a fertiliser.
The article quoted former EPA scientist David Lewis, who opposed the spreading of sludge on cropland in the mid 1990’s.
“Spending billions of dollars to remove hazardous chemicals and biological wastes from water, only to spread them on soil everywhere we live, work and play defies common sense,” he was reported to have said.
It should be noted that the article referred to the US specifically and Class B biosolids. In the UK, at least, this sludge would not be permitted to be spread on land.
Scientists from the U.S. Geological Survey (USGS) reported that they also found that biosolids contain relatively high concentrations (hundreds of milligrams per kilogram) of the active ingredients commonly found in a variety of household products and drugs.
They concluded that what isn’t known is what this means in terms of environmental impact, and the next step would be to look at the transport and fate of these substances once the biosolids are applied to land.
It is important to be aware of the emerging areas of concern with biosolids, but that currently they have benefits to land application due to nutrient content and soil conditioning properties. Monitoring and risk assessments, as well as appropriate application rates to land, will be important in ensuring that the most appropriate use of biosolids is made.
- Aquatech Online would like the thank Karyn Georges, head of consulting for Isle Utilities in the UK, for her support in producing this article.