Sludge, biosolids & bioresources: Our essential guide
Table of contents
- Sludge: Turning waste into a valuable resource
- What is sewage sludge?
- Wastewater sludge process
- Sludge pumps: designed and built to be tough
- What are biosolids?
- Biosolids fertiliser
- The future of biosolids
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
Sludge processing, treating sewage sludge for reuse, aims to minimize volume and pathogens, ensuring a stable product for land recycling.
A secondary goal is to produce biogas for energy and recover valuable resources like phosphorus as struvite. Various treatment methods, including thickening, dewatering, and steps like anaerobic digestion, composting, or thermal treatment, are utilized for these purposes.
The sewage sludge treatment process can include the three stages of thickening, digestion and dewatering processes, detailed below.
Thickening, a crucial step in wastewater treatment, occurs both before and after digestion and dewatering processes. According to Suez, thickening reduces sludge volume, optimizing conditioning, stabilization, and dewatering while minimizing facility size and operating costs. Methods like dissolved air flotation and rotary drum thickening are commonly used, yielding sludge with 2-8% dry solids before dewatering, typically increasing to 20-35% after.
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 it 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: anaerobic digestion reduces sludge volume by breaking down biomass without oxygen, yielding biogas, particularly valuable as utilities grapple with rising sludge production and disposal challenges. Anaerobic microorganisms convert organic matter into methane, with mesophilic (36 °C) and thermophilic (50 or 55 °C) options impacting 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 consist 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.
The Environmental Protection Agency (EPA) defines biosolids as nutrient-rich organic materials derived from treating domestic sewage. These residuals, once processed, can be recycled and used as fertilizer to enhance soil productivity and promote plant growth.
Differentiating between biosolids and sludge varies by region. In the US, biosolids are the product of sewage treatment, while sewage sludge refers to untreated solids from the wastewater treatment process. In contrast, in Germany, sludge is applied to land.
The term biosolids was introduced in the early 1990s to distinguish treated sludge approved for land application.
According to the Australian Water Association biosolids may contain macronutrients like nitrogen, phosphorus, potassium, and sulphur, along with micronutrients such as copper, zinc, calcium, magnesium, iron, boron, and manganese.
Biosolids fertiliser
Severn Trent Water states that biosolids, containing essential crop nutrients like nitrogen, phosphate, and sulphur, can supplement or even replace manufactured fertilisers.
EPA-certified biosolids, when applied at regulated rates, have been proven to significantly enhance crop growth and yield in agriculture.
Biosolids improve soil quality by enhancing workability and drainage in heavy soils and increasing water-holding capacity in lighter soils, reducing drought risk. Additionally, they enhance soil biological activity and fertility.
Biosolids quality varies based on wastewater source, treatment procedures, and dewatering methods.
Furthermore, the storage time ahead of land application can also make a big difference to nutrient concentration.
In the US, about half of all biosolids produced are utilized on less than one percent of the agricultural land. In the UK, approximately 80% of biosolids are recycled to land, contingent upon population density and available arable land.
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
Increased attention has focused on the environmental and public health implications of sludge and biosolids. As wastewater treatment plants must eliminate various substances like metals and pharmaceuticals, some inevitably end up in the sludge.
Initial UK research suggests microplastics in sludge, unsurprising given high removal rates in wastewater treatment plants.
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.
Certain countries restrict biosolids land application due to concerns about long-term environmental effects from heavy metals and organic contaminants. However, public pressure often influences these restrictions more than scientific data.
Within the US, there has been some attention of the human health impact of applying lower quality biosolids to land.
In 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-1990s.
“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..
While emerging concerns about biosolids are important, their current benefits in land application, including nutrient content and soil conditioning properties, are notable. Effective monitoring, risk assessments, and appropriate land application rates are crucial for ensuring the optimal use of biosolids.
- Aquatech Online would like the thank Karyn Georges, head of consulting for Isle Utilities in the UK, for her support in producing this essential guide.
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