INTRODUCTION:

Sludge recovery from WWTP is an important aspect of the wastewater treatment process. Sludge, also known as biosolids, is a byproduct of the treatment process that contains organic matter, nutrients, and other contaminants. In the past, sludge was often disposed of in landfills or incinerated, leading to environmental and public health concerns. However, with the increasing recognition of the value of sludge as a resource, there has been a shift towards sludge recovery and reuse. Sludge can be used as a fertilizer for agricultural purposes, a soil conditioner, or a source of energy through processes such as anaerobic digestion or thermal treatment^1–4.

Sludge recovery not only allows for the reuse of valuable resources, but it also reduces the amount of waste produced by WWTP. This has significant environmental and economic benefits, as it reduces the demand for landfill space and reduces the costs associated with sludge disposal³. In addition, the recovery and reuse of sludge can contribute to the circular economy by closing the nutrient loop and reducing the reliance on external inputs for agriculture ^5,6.

Sludge recovery plays a crucial role in the wastewater treatment process as it allows for the reuse of valuable resources and the reduction of waste. The recovery and reuse of sludge can help to close the nutrient loop, reducing the reliance on external inputs for agriculture and contributing to the circular economy⁷. In addition, the use of sludge as a fertilizer or soil conditioner can help to improve soil quality and increase crop yields⁸. Sludge recovery can also have economic benefits, as it can reduce the costs associated with sludge disposal and increase the revenue generated through the sale of recovered products^9,10. For example, the energy generated through processes such as anaerobic digestion can offset the costs of the treatment process and generate additional income¹¹. Furthermore, sludge recovery can help to mitigate environmental and public health concerns associated with the disposal of sludge. Landfilling or incineration of sludge can release greenhouse gases and contribute to air pollution, as well as posing a potential risk to human health through the leaching of contaminants into the environment^12,13. By recovering and reusing sludge, these negative impacts can be reduced or avoided. Overall, the recovery and reuse of sludge from WWTP is an important aspect of the treatment process that can have significant environmental, economic, and societal benefits.

Sludge recovery from WWTP is a complex and multifaceted process that involves the treatment and management of sludge to enable its reuse or disposal. In this review, we aim to provide a comprehensive overview of current practices and tips for sludge recovery from WWTP.

Overall, the objectives of this review are to present a comprehensive review of current practices and tips for sludge recovery from WWTP and to offer practical insights for improving the efficiency and sustainability of this important process. “We conducted a search for relevant studies on the topic of our article using several data collection techniques including Springer, Scopus, ScienceDirect, and Web of Science via the e-resources platform of the “Moroccan Institute of Scientific and Technical Information” (IMIST) under the “National Centre for Research in Science and Technology” in Morocco. These platforms identify recurring research conducted globally, which provides Moroccan researchers with current insights on how to understand and investigate this topic. The search was based on specific keywords such as Sludge recovery, WWTP (WWTP), primary sludge, secondary sludge, tertiary sludge, sewage sludge and its chemical composition. The collected articles were then analyzed, and the results were compared”^14,15.

Types of SPTP “sludge produced in the treatment process”

Primary sludge:

Primary sludge is a type of sludge produced during the physical treatment process of wastewater. It is generated through the removal of suspended solids and organic matter from the wastewater through processes such as sedimentation or filtration. Primary sludge is typically characterized by a high concentration of organic matter, suspended solids, and nutrients, as well as a high moisture content¹¹.

Secondary sludge:

Secondary sludge is a type of sludge produced during the biological treatment process of wastewater. It is generated through the removal of organic matter and nutrients from the wastewater through processes such as aeration or nitrification¹². Secondary sludge is typically characterized by a lower concentration of organic matter and a higher concentration of nutrients compared to primary sludge¹¹.

Tertiary sludge:

Tertiary sludge is a type of sludge produced through advanced treatment processes of wastewater. It is generated through the further removal of organic matter and contaminants from the wastewater, resulting in a higher quality sludge compared to primary and secondary sludge. Tertiary sludge is typically characterized by a lower concentration of organic matter and contaminants and a higher concentration of nutrients compared to primary and secondary sludge.

Primary, secondary, and tertiary sludge possess the potential for utilization as fertilizer or soil conditioner, as it contains nutrients that can help to improve soil quality and increase crop yields¹¹. However, the high moisture content and potential for the presence of contaminants can limit the direct application of sludge to agricultural land. As such, sludge may need to be treated or stabilized before it can be used as a fertilizer¹⁶. Tertiary sludge may also have potential as a feedstock for anaerobic digestion or thermal treatment processes, as it has a higher quality compared to primary and secondary sludge. However, the potential for the presence of contaminants may still require tertiary sludge to be treated or stabilized before it can be used as a fertilizer or feedstock. The composition of sludge can vary depending on the type of wastewater being treated and the treatment processes used. For example, primary sludge from a municipal WWTP may contain a higher concentration of organic matter compared to primary sludge from an industrial WWTP^17,18. In addition, the characteristics of primary sludge can be influenced by factors such as the pH, temperature, and retention time of the treatment process^19,20. However, the potential for the presence of contaminants may still require tertiary sludge to be treated or stabilized before it can be used as a fertilizer or feedstock.

Methods for sludge recovery:

Thickening:

Thickening is a process used to increase the solids concentration of sludge in order to reduce the volume and improve the handling and transportability of the sludge. There are various thickening methods available, including gravity thickening, flotation thickening, and centrifugation¹².

Gravity thickening involves the use of sedimentation tanks or basins to separate the solids from the liquids in the sludge through the action of gravity. The solids are allowed to settle to the bottom of the tank, where they can be removed and further treated or disposed. Gravity thickening is a simple and cost-effective method, but it is limited by the low solids concentration that can be achieved¹⁷.

Dewatering:

Dewatering is a process used to remove water from sludge in order to reduce the volume and improve the handling and transportability of the sludge. Dewatering can be achieved through the use of mechanical, chemical, or thermal processes, or a combination of these methods.

Mechanical dewatering involves the use of equipment such as belt filters, centrifuges, or screw presses to separate the solids from the liquids in the sludge through the action of mechanical forces. Chemical dewatering involves the use of chemicals such as polymers or inorganic coagulants to improve the dewatering performance of sludge through the formation of flocs or the destabilization of the sludge. Chemical dewatering can be effective, but it may require careful management to ensure the safety and environmental sustainability of the process. Thermal dewatering involves the use of heat to evaporate water from the sludge, either through the use of steam or hot air. Thermal dewatering can achieve high solids concentrations, but it is energy-intensive and may generate emissions that can impact the environment.

Drying:

Drying is a process used to remove water from sludge in order to reduce the volume, improve the handling and transportability, and increase the stability and shelf life of the sludge. Drying can be achieved through the use of mechanical, thermal, or solar energy, or a combination of these methods. Mechanical drying involves the use of equipment such as rotary dryers, fluidized bed dryers, or spray dryers to remove water from the sludge through the action of mechanical forces. These methods can achieve high solids concentrations, but they are energy-intensive and may require the use of heat or other drying agents to improve the drying performance. Thermal drying involves the use of heat to evaporate water from the sludge, either through the use of steam or hot air. Thermal drying can achieve high solids concentrations, but it is energy-intensive and may generate emissions that can impact the environment²¹.

In general, every approach to sludge recovery comes with its own set of pros and cons. The optimal method choice will hinge on factors such as the specific traits of the sludge and the objectives of the treatment process.

Tips for optimizing sludge recovery:

Proper sludge management:

Proper sludge management is an important aspect of optimizing sludge recovery from WWTP. Effective sludge management can help reduce the volume and environmental impacts of sludge disposal, while improving the efficiency and sustainability of the treatment process.

There are several key considerations for proper sludge management:

Collection and transportation:

Proper collection and transportation of sludge can help reduce the risk of spills or leaks, which can have negative environmental impacts. This can include the use of appropriate containers, vehicles, and handling techniques to minimize the risk of accidents or releases.

Storage:

Proper storage of sludge can help reduce the risk of odors, pests, or environmental contamination. This can include the use of covered storage facilities, temperature control, and effective ventilation to minimize the risk of emissions or odors.

Treatment and disposal:

The “treatment and disposal” of sludge should be carried out in a manner that is safe, environmentally-friendly, and compliant with all relevant regulations. This can include the use of appropriate treatment technologies such as anaerobic digestion, composting, or land application, as well as the management of any residuals or by-products that may be generated during the treatment process.

Incorporation of energy-efficient technologies:

Sludge recovery from WWTP is an important aspect of optimizing the treatment process and reducing the environmental impact of wastewater treatment. One effective method for improving sludge recovery is the incorporation of energy-efficient technologies.

One such approach involves employing anaerobic digestion, a process that breaks down organic matter without the presence of oxygen. Research has demonstrated that anaerobic digestion substantially enhances the energy efficiency of sludge treatment while simultaneously decreasing greenhouse gas emissions and the overall volume of sludge generated²². Another energy-efficient technology for sludge recovery is the use of thermal hydrolysis, which involves the use of heat and pressure to break down organic matter and improve the dewaterability of sludge. Thermal hydrolysis has been demonstrated to increase the energy efficiency of sludge treatment and improve the quality of the resulting biogas. The use of membrane filtration technologies, such as microfiltration and ultrafiltration, can also improve the energy efficiency of sludge recovery by reducing the amount of energy required for dewatering. These technologies have been shown to be effective in removing solids and contaminants from sludge, resulting in a more concentrated and easily dewatered sludge product²³.

Production and recovery of sewage sludge by the EU:

In 2016, the European Commission reported that “almost 8 million tons of sludge were generated by wastewater treatment, with agriculture being the primary sector for its reutilization in most EU member states; The application of sewage sludge in agriculture is governed by Council Directive No. «86/278/EEC», which aims to encourage the utilization of biosolids while implementing regulations to safeguard against adverse impacts on soil, animals, vegetation, and humans. The regulations established by the EU commission entail prohibiting untreated sludge and outlining precise guidelines for the sampling and analysis of both sludge and soil. It also requires keeping detailed records of sludge quantities produced, utilized in agriculture, composition and properties, treatment type, and usage locations. The directive outlines several parameters, including chromium, cadmium, copper, nickel, lead, zinc, and mercury in milligrams per kilogram of dry matter. Additionally, the analysis should include parameters such as organic matter, pH, nitrogen, and phosphorus”²⁴. To ensure public health and safety in regards to human consumption, the Commission Regulation (EC) No. 1881/2006 of 19 December 2006, sets out maximum levels for specific contaminants in food products to maintain toxicological levels that are deemed acceptable²⁵. As per an EU assessment roadmap concerning the Evaluation of the Sewage Sludge Directive «86/278/EEC», published in June 2020, using sewage sludge as an alternative to chemical fertilizers, particularly phosphorus fertilizers, is an effective solution. The “European Green Deal and Circular Economy Action Plan” (CEAP) prioritize material recycling in line with circular economy principles. However, it is crucial that recycled resources are not contaminated, as this would increase pollution in soil, water, and air, contradicting the “Commission's zero pollution ambition” announced in the “European Green Deal”. A strategy for achieving this ambition is expected to be released in 2021.

Figure 1: The mean “production and disposal of sewage sludge from urban wastewater” across 22 European countries between 2009 and 2020 ²⁶

In these 12 years, we notice that the production of sludge in the EU is very high (Figure) and it varies between 44,56 thousand tons in 2020 and 314,52 thousand tons in 2010, which represents the highest quantity generated by the WWTP in the last years. At the same time, the total amount of sludge disposed of is very important, as sludge destined for agricultural use represents a high amount, varying between 15.47 thousand tons in 2020 and 142.75 thousand tons in 2010 (Figure 1). The variation in percentage between years may have various causes, including the implementation of new directives and legislation. For instance, the slow reduction in percentage observed from 2009 could be due to the introduction of the waste hierarchy under Directive 2008/98/EC²⁷. However, in 2015, the percentage began to increase again, which could be related to the European Commission's complete rethinking of the waste management strategy in recent years. This rethinking included the adoption of a “Circular Economy Action” Plan in 2015, This presents a detailed and ambitious plan of action that covers the entire lifecycle, spanning from production and consumption to waste management and the secondary raw materials market. Additionally, a revised legislative proposal on waste was also introduced. These observations, coupled with the percentage of sewage sludge used in agriculture, are significant in evaluating the quantity of biosolids that are directly applied for agricultural purposes in the EU.

Sludge nutrients composition:

Sludge is frequently employed as a soil enhancer due to its richness in vital nutrients necessary for plant development. The nutrient composition of sludge may differ based on its source and the treatment process utilized. It typically comprises macronutrients like nitrogen, phosphorus, and potassium, along with micronutrients such as zinc, copper, and manganese. The amount of nutrients in sludge can be modified by treatment techniques such as anaerobic or aerobic digestion, centrifugation or thermal drying. The use of sludge as a soil amendment can therefore be an economical and environmentally friendly option for farmers, especially in areas where traditional sources of fertilizer are limited or expensive. However, the use of sludge must be regulated to minimize the risk of soil and water contamination²⁸.

The table 2 lists the nutrient composition of various types of sludge obtained from different countries. The results indicate significant variations in the levels of nutrients, particularly phosphorus, calcium, nitrogen, magnesium, and potassium, in different types of sludge across countries. Nitrogen levels ranged from 2.7g.kg−1 DM in Argentina to 56.6g.kg−1 DM in China. Similarly, phosphorus levels ranged from 0.586g.kg−1 DM in Morocco to 33g.kg−1 DM in Denmark. Furthermore, the results indicate that sludge composition differs significantly based on its source and whether it is digested, pure, or mixed. For instance, digested sludge had higher nitrogen levels than pure sewage sludge.

Table 2: Sludge nutrients composition

Parameters/ Country	Azote (N)	Carbon (C)	Total Phosphorus (TP)	Potassium (K)	Calcium (Ca)	Magnesium (Mg)	References
Parameters/ Country	g.kg−1 DM						References
Argentina	2,7 ± 0,47	32,4 ± 5,1	7,7 ± 1,4	1,4 ± 0,3	-	-	²⁹
Brazil 1997 1998 1999 2000 2001 2022	6,4 37,31 28,72 28,94 36,75 34,08	- - - - - -	3,32 11,3 17,41 15,58 15,54 21,62	0,97 1,7 1,47 1,85 2,74 1,9	- - - - - - -	- - - - - - -	³⁰
China	51,2	-	5,51	-	-	-	³¹
China	56,6	334,5	9,8	-	-	-	³²
Denmark	47	327	33	-	-	-	³³
Finland PSS DS	31,00 7,40	-	26,0 9,90	2,10 1,10	38,00 -	- -	^34,35
India	17,3± 0,2	-	0,717± 0,06	0,209± 0,002	-	-	³⁶
India	18	-	16,1	1,83	-	-	³⁷
Morocco	52,2± 2	-	0,586± 0,018	0,920 ± 0,021	26,1 ± 1,4	12,9 ± 0,7	³⁸
Pakistan	6	-	13	13	-	-	³⁹
Pakistan DAS DASB PAS PASB MS MSB	14,5 4,3 14,6 5,4 21,2 6,4	- - - - - -	13,33 13,03 13,38 12,44 12,4 13,87	- - - - - -	- - - - - -	- - - - - -	⁴⁰
Poland	14,5	690	3,5	-	-	-	⁴¹
Poland	15,7	-	13,3	-	>111,9	-	⁴²
Spain	22,20	-	16,60	4,70	-	-	⁴³
Spain	20,00	190,00	-	0,8	35,6	2,9	⁴⁴
Spain	2,48	3,16	5,62	7,89	38,5	2,65	⁴⁵

DS: “Digested sludge”; PSS: “Pure sewage sludge”; DAS: “Disposal area sludge”; DASB: “Disposal area sludge biochar”; PAS: “Populated area sludge”; PASB: “Populated area sludge biochar”; MS: “Mixed sludge”; MSB: “Mixed sludge biochar”

Table 3: The FAO, WHO and the «86/278/EEC» Directive and standards for heavy metal concentrations in soil ²⁴

	WHO “Maximum Permissible Pollutant Concentrations in the Receiving Soils (mg kg-1)”	FAO “Maximum Permissible Concentration of Potentially Toxic Elements in Soil (mg kg-1Dry Solids) pH = 5.0 < 5.5”	Directive «86/278/EEC» “Limit Values for Concentrations of Heavy Metals in Soil (mg kg -1of Dry Matter of Soil) pH= 6 to 7”
“Arsenic” (As)	08.00	50.00	-
“Cadmium” (Cd)	04.00	03.00	01.00 to 03.00
“Chromium” (Cr)	-	400 (prov.)	-
“Copper” (Cu)	-	80.00	50.00 to 140.00
“Fluoride” (F)	635.00	500.00	-
“Lead” (Pb)	84.00	300.00	50.00 to 300.00
“Mercury” (Hg)	07.00	01.00	1.00 to 01.50
“Molybdenum” (Mo)	00,60	04.00	-
“Nickel” (Ni)	107.00	50.00	30.00 to 75.00
“Zinc” (Zn)	-	200.00	150.00 to 300.00
“Silver” (Ag)	03.00	-	-
“Boron” (B)	01.70	-	-
“Beryllium” (Be)	00.20	-	-
“Barium” (Ba)	302.00	-	-
“Selenium” (Se)	06.00	-	-
“Antimony” (Sb)	36.00	-	-
“Titanium” (TI)	00.30	-	-
“Vanadium” (V)	47.00	-	-

Legislation:

Sewage sludge is an unavoidable byproduct of municipal WWTP operation, and its increasing production and associated disposal impact present a significant challenge in many countries ⁴⁶. Both FAO and WHO have established guidelines for sewage sludge use.

The report presents limit values for pollutants that were not included in the FAO document or EU directive, such as Boron (B), Silver (Ag), Titanium (Ti), Vanadium (V) and Beryllium (Be) (Table 3).

CONCLUSION:

In conclusion, the recovery of sludge from WWTP is a crucial aspect of environmental sustainability. Sludge is a byproduct of the wastewater treatment process and contains valuable nutrients and energy that can be recovered through various treatment processes. By carefully managing and optimizing these processes, it is possible to recover a significant portion of the nutrients and energy contained in sludge, reducing the amount of waste that must be disposed of in landfills.

There are several approaches to sludge recovery, including anaerobic digestion, thermal drying, and electrocoagulation. Each of these methods has its own advantages and disadvantages, and the most appropriate method will depend on the specific needs and resources of each facility. In order to optimize sludge recovery, it is important to consider factors such as the characteristics of the sludge, the treatment technologies available, and the potential end-use options. The statistics show that the quantity of sludge produced by the countries of the European Union is very important, and that the agricultural field represents a way of primordial valorization of these sludges, considering that these last ones are rich in nutrients such as Nitrogen, Phosphorus, Magnesium, Potassium... except that this reuse of the sludges in the agricultural field requires good practices and a law which frames this activity.

Overall, this review has provided a comprehensive overview of current practices and tips for improving the efficiency and sustainability of sludge recovery from WWTP. By implementing effective treatment processes and carefully considering the various options available, it is possible to significantly improve sludge recovery and contribute to a more sustainable future.

ACKNOWLEDGMENTS:

Thank to researchers at the LVAPAI in FSR-Kenitra.

CONFLICT OF INTEREST:

No.

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Received on 15.01.2024 Modified on 18.03.2024

Research J. Pharm. and Tech 2024; 17(8):4117-4124.

DOI: 10.52711/0974-360X.2024.00638

Method	Advantages	Disadvantages
Thickening	Reduces the volume of sludge, improving the handling and transportability of the sludge	Limited in the solids concentration that can be achieved, depending on the method used
	Can be simple and cost-effective, depending on the method used	May require the use of chemicals or energy to improve the thickening performance
	Can be suitable for a wide range of sludges	May generate emissions or waste streams that can impact the environment
Dewatering	Reduces the volume of sludge, improving the handling and transportability of the sludge	Energy-intensive, depending on the method used
	Can achieve high solids concentrations, depending on the method used	May require the use of chemicals or energy to improve the dewatering performance
	Can improve the stability and shelf life of the sludge	May generate emissions or waste streams that can impact the environment
Drying	Reduces the volume of sludge, improving the handling and transportability of the sludge	Energy-intensive, depending on the method used
	Can achieve high solids concentrations, depending on the method used	May require the use of drying agents or energy to improve the drying performance
	Can improve the stability and shelf life of the sludge	May generate emissions or waste streams that can impact the environment