**3. Legislative aspects in the use of sewage sludge in agriculture**

The management of SS in the EU is regulated by various legislative acts. The Directive 2008/98/EC establishes the fundamental ideas and terminologies, such as waste, recycling, and recovery [21]. It explains the basic concepts of waste management, the distinction between waste and secondary raw material ("end-of-waste criteria"), waste, and by-products. The directive lays down basic principles of waste management without adversely affecting human life, health, nature, and the environment. Waste legislation and policy of the EU Member States shall apply as a priority order with the waste management hierarchy (**Figure 1**).

An ex-post evaluation of the SS Directive 86/278/EEC in 2014 showed that its initial objectives were achieved, in spite of large variations in the amount of SS used in agriculture in the Member States (from none to well over 50%) [21, 22]. Currently (2020–2021), the EU initiated an evaluation of legislation efficiency, as well as the risks and opportunities of SS used in farming [23, 24].

Furthermore, two EU working documents on sludge have been produced: the EU Working Document on sludge (2000) and the EU Working Document on sludge and biowaste (2010). The EU Working Document on sludge (2000) indicates that to be used without restrictions, sludge should undergo an hygienization process by an "advanced treatment," which should result in at least a 6-log-unit reduction in *Escherichia coli*, as well as create a sludge that meets the following criteria: In 50 g, there is no Salmonella (wet weight, WW) and *E. coli* <500 colony-forming unit CFU g−1. It was also proposed that sludge produced by "conventional treatments" should show a 2-log-unit reduction of *E. coli*, and its use is allowed with restrictions on its application time, site, and modality. Mesophilic anaerobic digestions at a temperature of 35°C with a mean retention time of 15 days and thermophilic anaerobic digestions at a temperature of at least 53°C for 20 h as a batch, without admixture or withdrawal during the treatment, are indicated, among others, as conventional and advanced treatment processes, respectively. The more recent EU document only suggests the limited absence of Salmonella in 25–50 g and *E. coli* <5 × 105 g−1 WW as possible criteria for the use of sludge in agriculture [25].

According to the EPA Environmental guidelines published in 2000 on stabilization of biosolids products [26], a biosolids product must meet at least one pathogen reduction requirement and at least one vector attraction reduction requirement [27]. Stabilization Grade A includes thermally treated biosolids (at least 50°C), high pHhigh temperature process and biosolids from unknown processes, while stabilization Grade B—anaerobic digestion, aerobic digestion, air drying, composting, lime stabilization, extended aeration, and other processes accepted by the EPA products [26].

#### **Figure 1.**

*The Waste Framework Directive 2008/98/EC priority order with the waste management hierarchy [21].*

*Application of the Sewage Sludge in Agriculture: Soil Fertility, Technoeconomic, and Life-Cycle… DOI: http://dx.doi.org/10.5772/intechopen.104264*

### **4. Economical aspects: technological efficiency and circular economy**

In the context of sustainable development principles some main components, which determine the rational solution of the multi-faceted problem of municipal SS, must be considered. Poor farming practices combined with the overuse of chemical fertilizers on poor soils have caused a negative environmental impact, which leads to the degradation of arable land. The effort to increase productivity by increasing the use of various chemicals in fertilizers further diminishes soil fertility. With each harvest, the soil loses organic compounds, and permanent aggravation of improper agricultural practices often prevents the land from recovering. World chemical fertilizer consumption increased from 70.95 kg/ha in 1976 to 138.16 kg/ha in 2016. And in some regions, the fertilization dose increased up to hundreds and even thousands of kilograms per hectare (**Figure 2**) [28].

The quantity of organic elements in the soil constantly decreases. A significant part of the SS does not return to the soil, but is disposed into the sea, is incinerated, or is subject to other different kinds of destructive effects, leading to drastic decreases in soil fertility and continuous soil degradation.

The quantity of the SS constantly increases. The peculiarity of SS lies in its multimineral compound and a huge range of organic matter; in fact, the SS is a nitrogenphosphorus-potassium organic fertilizer, containing a full set of microelements necessary for the growth of crops. However, due to the high risk of pathogenic impact, a huge part of human and material resources is directed to the destruction of this important resource.

The overwhelming majority of the SS disposal methods are expensive, harmful, or contain both factors. Most municipalities face the growing problem of wastewater treatment. In many cases, waste is dumped into landfills, oceans, or incinerated. The rational solution to the problem of municipal SS disposal lies in an integrated approach to returning the sludge into the agricultural cycle [29].

The directive introduces the "polluter pays" principle and the extended producer responsibility. Some existing projects of producing energy, for example, biogas, minerals, and chemicals out of the sludge, do not prove to be sustainable and viable financially. Furthermore, in most cases, most of the sludge is eventually dumped at the end of the process. Incineration represents the total elimination of the sludge but is extremely expensive. It seems to be the most rational to consider SS not as a problem, but as a valuable resource.

In recent years, out of concern for the profound soil degradation, a growing trend of shifting to organic fertilizers is taking over within the agricultural industry.

The global fertilizer market was valued at around \$360 billion before the COVID-19 pandemic with organic fertilizer making up just \$6.8 billion. The organic fertilizer market is described as steadily increasing and expected to post a CAGR (Compound Annual Growth Rate) of 14% during the period 2019–2023, with the key factor being increased food demands and agricultural shortages due to population growth and climate change [30].

In case of the continuing negative influence of the high transport, logistics, and energy costs, the SS processing can offset the lack of fertilizers through a domestic product that costs only a fraction of the price to make, creating a local commodity with a considerable economic edge.

Sewage sludge is a natural epidemic focus, and the detection of SARS-CoV-2 in fecal masses led to the long-overdue conclusion to strengthen human health protective measures and counteract the emergence of epidemics [31]. The necessity of the

**Figure 2.**

*Fertilizer consumption by different countries and regions. (a) Data on some countries and regions with fertilizer consumption below 500 kg/ha; (b) data on countries with rapid growth of fertilizer consumption, which exceeded 500 kg/ha [28].*

implementation of new biological safety criteria can have a significant economical and long-term structural influence on the development of the entire sphere of processing and use of SS. For instance, regarding the sediment formed during the epidemic, it is recommended to avoid its traditional aerobic composting. At once, in the sludge undergoing thermal disinfection treatment, the risk of infection with SARS-CoV-2 is considered in the range from low to negligible [32]. Intensive decontamination measures will make the product more expensive, but more in line with the requirements of sustainable development.

To prevent potential biological threats toward the environment and human health, it becomes increasingly important to develop the most isolated from the environment hermetic methods for the SS disposal, without destroying the organic component, valuable for agriculture.

Economic aspects of SS hygienization have been analyzed [33]. The energy requirement per 100 tons of sludge was estimated depending on different disinfection conceptions. Thus, solar dehydration and chemical treatment with alkali consume 11.7 and 148.3 kW h with the production of 80 tons and 99.6 tons, respectively. In turn, the most expensive technology is gamma irradiation, which consumes 64,800 kW h for obtaining 97.6 tons of the product. The thermal drying also requires quite a high energy consumption, that is, 21,000 kW h for 20 tons of product. The composting does not consume electricity [33]. The high costs of thermal hydrolysis and ultrasonic methods and the need for a neutralizing agent in acid solubilization limit the rapid implementation of these processes in industrial practice [34].

Our testing of the infrared heating method for SS disinfection demonstrated successful results. It took 15 min for the material with an 80% humidity, including the time it required to heat the layer to 95°C, which is below the temperature at which the organic matter decomposes [35].

The widespread usage of SS biomethanation has resulted in the building of a number of complex installations that combine biological wastewater treatment facilities with anaerobic digesters. The development of digestate-derived granulated soil fertilizers is based on physicochemical processing of biostabilized sludges, in keeping with the circular economy concept and the concept of "waste-to-product" [36].

In this respect, the costs of pretreatment technologies for SS biomethanation with further conversion of digestate to fertilizers should be taken into consideration. *Application of the Sewage Sludge in Agriculture: Soil Fertility, Technoeconomic, and Life-Cycle… DOI: http://dx.doi.org/10.5772/intechopen.104264*

#### **Figure 3.**

The estimated energy utilized for the mechanical operations during SS disintegration and anaerobic digestion (stirring and pumping) was calculated to be 1253.6 kW h per ton [37]. The energy spent for SS pretreatment may vary depending on the solubilization [38], used consumables [39], and methods [40]—thermochemical (TC), sonic, thermo-chemo-sonic, etc. It is experimentally proven that combined disintegration pretreatment should be more efficient. The energy consumption for TC sludge pretreatment (30% solubilization) for biogas production was calculated to be 1588.552 kW h per ton of sludge. The thermos-chemo-ozone (TCO3) pretreatment can optimize the total energy input up to ~721.766 kW h per ton [41].

The evaporation of water should be weighed out between the energy costs in the process and the SS management costs without drying [42].

According to the economic feasibility review of our project for fast SS recycling into biological fertilizers, the energy cost will be nearly \$30 per ton of fertilizers (with its humidity ~50% and energy costs \$0.1 per kW h and initial SS humidity ~80%). The tested method allows providing 1 ton of bio-pathogenic-free fertilizers due to utilizing up to 1.5 tons of SS and withal avoid other SS disposal costs (**Figure 3**) [43].

The applied methods and technical decisions have international priority under the Paris Convention for the Protection of Industrial Property, the World Intellectual Property Organization (WIPO) Eurasian Patent Organization (EAPO) and national patent organizations.

### **5. Sewage sludge treatment technologies**

#### **5.1 Stabilization**

Stabilization of SS aims at reducing some disadvantages of SS (e.g., odor, leaching of heavy metals, etc.), thus considerably extending the potential of SS application. The extent to which readily biodegradable organic matter has degraded is referred to as the degree of stability [44]. Mixing of SS with fly ash, lime, peat, clay, straw, and

other residues considerably improve SS characteristics, reducing leachability for metals and soil loss [45, 46]. The addition of wheat straw to the bioaugmented SS after 16 days incubation demonstrated the highest and most stable respiration intensity, the lowest ammonia emission, and the highest stimulation effect on the cress seedling growth, as compared to other treatment types [47].

Santos et al. [22] compared the performance of six residues serving for (i) sludge drying and (ii) improving agronomic properties of the final product. Weathered coal fly ash, bottom biomass ash, green liquor dregs, lime mud, eggshell, and rice husk were chosen as adjuvants based on circular economy and industrial ecological parameters. (0.15 g adjuvant/g SS wet basis). The addition of bottom biomass ash to SS promoted the highest diffusion coefficient and drying rate. The highest positive effect on agronomic parameters was shown for the SS amended with eggshell. Among evaluation criteria were acid neutralization capacity, oxygen uptake rate, and germination index [22].
