Transforming sewage sludge into agricultural fertilizer

Iguá was born in 2017 and is now present in 39 municipalities of Brazil, with the objective of bringing sanitation services to the populations served and helping the country achieve universal water and sewage treatment services (1).

The project responds to the need to find sustainable alternatives for the disposal of sludge generated in the concessionaire's sewage treatment plants. It is part of the company's sustainability strategy, which is based on four pillars: water security, efficiency in water production and distribution, responsibility in sewage collection and treatment, and respect for people (2).

The sanitary sewage treatment process produces sewage sludge as a by-product. This material, when managed as solid waste, needs to be disposed of correctly to prevent soil and water contamination. In Brazil, the main destination for this waste is still landfills. 

In 2022, Iguá's operation in Cuiabá (Águas Cuiabá) launched a project to transform sewage sludge into agricultural fertilizer, in partnership with local research centers and in alignment with the granting authority. 

Sewage sludge goes through a series of physical-chemical and biological treatments to prepare it for use in soil. In Brazil, the entire process is governed by strict environmental legislation, and batches are tested and approved via specific laboratory analyses. 

• Step 1: Receiving and storing the sludge – in an appropriate location, impermeable windrows are formed to receive the sludge from the WWTPs. 

• Step 2: Batch formation – in the test developed, each windrow has a capacity of approximately 50 tons of sludge and is called a batch. So, one windrow equals one batch of sludge. From this, the dosage of quicklime is calculated. The process follows Brazil’s CONAMA legislation Nº498/2020 (3), which guides the use of sludge in agriculture and contains a disinfection methodology called Prolonged Alkaline Stabilization. 

• Step 3: Mixing with quicklime – with the amount of lime calculated, the sludge liming stage is carried out when the windrow is full of sludge and a batch is completed. 

• Step 4: Batch maturation – after quicklime mixing, the batch maturation phase lasts around 45 days. 

• Step 5: Sampling and Characterization - after batch maturation, the biosludge formed is sampled. This sampling is sent to the laboratory for characterization according to CONAMA 498 parameters. At this stage, the sludge is deemed either CLASS A (better) or CLASS B (with more restrictions on use).  

• Step 6: Release and Shipping for Agriculture - after release and validation in laboratory analyses, the biosludge is ready to be delivered to properties. For the delivery to be made, the farmer is chosen in advance, a soil sample is collected from their area for characterization, and the agronomic recipe is prepared to calculate the quantity needed for that property. The recipe must be signed by an agronomist. 

Figure 1: Stages of bench-scale sludge treatment.

The first property to receive biosludge through Iguá’s project is in a rural region of the municipality of Cuiabá, called URT do Leite. Two areas of application of the biosludge were defined in accordance with the agronomic projects of the site, which was a small farm producing dairy products. It used the biosludge to recover areas of sugar grass pasture and feed dairy cattle. 

Figure 2: Selected areas

In the first application area, over a period of three months, there was a gain in plant mass of around 70%, in addition to a visible increase in the height of açu grass. Furthermore, there was a 150% increase in milk production from cows that consumed sugar grass in the area that received the biosludge.

In relation to the second area, the biosludge was applied to recover a brachiaria grass pasture. After three months of application, there was a mass gain of around 60% among the clumps. 

All biosludge production was paid for by Iguá. 

Climate impact  

Targeted emissions sources 

Scope 3, solid waste category 

Decarbonization impact 

The disposal of sewage sludge to landfills in the Cuiabá region has an impact of around 0.7tCO2e emitted for each ton of sludge landfilled. A significant reduction in emissions is expected, with the additional generation of a positive impact on the local economy.

Business impact

Benefits 

Among the benefits, the following can be mentioned: 

• Quality: The biosludge applied to the pastures increased the quality, size, and color of the açu grass, directly reflected in the milk production of the cows that consumed the grass 

• Innovation: The innovation of this process can be highlighted, considering it is the first project for the sustainable use of WWTP sludge in the state of Mato Grosso 

• Cost savings: For Iguá, the study resulted in a potential cost reduction of around 6%, compared to the cost of sending waste to landfills.   

Costs 

The project development costs are: - Investment in the physical-chemical and biological treatment process of sewage sludge: area, chemicals, technical team - Investment in sample collection and analysis - Costs of transporting the material between the generation and final application site - Environmental licensing costs  

Indicative abatement cost 

The project has been a test, with no assessment of scalability costs yet  

Impact beyond climate and business

Co-benefits 

As the project is socio-environmental in nature, there are benefits related to supporting small-scale farmers. The project increases the income of these producers, with an increase in grass production and greater milk production by cows, strengthening family farming in the region. Social benefits are also highlighted, as the biosludge is delivered free of charge to low-income farmers. 

Furthermore, the cycle of waste generated in sewage treatment, which is sludge, is closed. In addition to being used for another purpose as raw material, this material is no longer sent to landfill, contributing to an increase in the useful life of this landfill.

The project promotes the development of the following SDGs: 6 and 12, to enable the sustainable use of a by-product from ETEs; 8, to favor the local economy; 11, to favor, through sustainable use (biosludge), the local production chain; 14 and 15, to reduce the need for pesticides that end up in bodies of water; 17, an integrated action between Águas Cuiabá, the Regulatory Agency, the City Hall, and small farmers. 

Potential side-effects 

As a potential collateral effect of the project, there is the risk of a negative effect from the use of sludge in some areas. Sewage sludge is a waste that, if poorly managed, can result in soil and water contamination, in addition to potential adverse effects of vegetation contamination, due to its richness in nutrients, and the possible presence of disease vector organisms.

However, this risk is mitigated by carrying out analyses of both the property's soil and the applied sludge. The entire process is controlled with laboratory reports and technical responsibility notes managed by professionals duly qualified for the role. Quality control at each stage of the process is essential for the successful transformation of waste into a co-product. 

Typical business profile  

The project can be implemented by organizations that generate sewage sludge – mainly companies in the basic sanitation sector located in regions with a developed agricultural sector, so that there is a balance between supply and demand for agricultural inputs. 

Approach 

The solution can be implemented in other locations that generate sludge in WWTPs. What must be considered, in addition to what was already mentioned above, is the methodology for drying and disinfecting the sludge, as well as all local legal and regulatory requirements. 

Stakeholders involved  

The project was developed through a partnership between the granting authority, Cuiabá City Hall, in its environment and sustainable development and agriculture departments, and the city's water and sewage service provider, which is Águas Cuiabá. Furthermore, the partnership with local farmers was extremely important. Internally, the project was the result of joint work between the areas of Sewage Treatment Management, Environmental Management, and Quality, Communication and Management. 

Key parameters to consider

• Solution maturity: The development of fertilizer from sewage sludge is a technology already known in the sanitation sector, but with different levels of application maturity 

• Technical constraints or pre-requisites: Sewage sludge must have specific technical characteristics that meet legal and regulatory requirements – chemical, physical, and microbiological characteristics of the raw material, quality of the treated material, type of crop in which the material is applied, environmental and agronomic licensing, and authorization from local management bodies, among others. The involvement of agronomic and sanitation specialists is necessary to guarantee the environmental and sanitary safety of the process. 

• Additional specificities: Brazil has, in its federal legislation, specific regulatory requirements for the use of domestic sewage sludge as fertilizer. Therefore, local legal requirements must be considered. 

• Other: Regional economic characteristics must be considered to ensure balance between the supply of biosludge and the demand for fertilizer. 

Implementation and operations tips

The process is in the testing phase, and there are still a series of steps to be completed. The operation has launched a new testing stage, with a greater quantity of biosludge and in areas with different crops. Operationalization of the system on a larger scale will, in the future, involve the construction of a sludge management unit, which can even process sludge from water treatment plants. The tests are supporting further technical and financial feasibility studies. 

The issue of scale is the main challenge for the implementation of a project like this, including the quantity of sludge generated, frequency of generation, biosludge development time, proximity and availability of areas for agricultural application, and necessary investment, among others. 

The process must always be developed in partnership with local entities, public authorities, research centers, and farmers.