**6.2 Shift in soil microbial community structure after application of the sludge-derived fertilizer**

When SS is applied to soil, it causes changes in the structure and functioning of the agroecosystem. The most sensitive component is the soil microbiota, which can undergo both stimulatory and inhibitory changes in the activity and structure. These changes are greatly dependent on soil characteristics and SS application rate.

The microcosm experiment with SS-amended sandy soil (25.71 g SS/kg dry soil) after 119 days has revealed significant changes in prokaryotic community composition at the phylum level, as compared to the non-amended control [48]. Specifically, in SS-amended soil, the relative abundance of Firmicutes reduced from 58.6% at Day 0 to 18.7% at Day 119, while Proteobacteria increased from 15.5% to 36.4%, respectively [69]. In the control soil, these two respective phyla did not change considerably for 119 days. The relative abundance of Actinobacteria in SS-amended soil has increased from 3.1% to 13.2%, while in the control soil decreased from 27.6% to 19.4% [69].

The use of sludge as a soil amendment has been shown to increase the activity of soil enzymes, for example, arylsulfatase, acid phosphatase, and alkaline phosphatase. Basal respiration and the fluorescein diacetate hydrolysis activity increased with increasing the dose of SS [70]. Changes in urease activity by soil microorganisms can be discussed in two aspects. First, urease activity reflects the activity of microorganisms involved in the nitrogen cycle in soil [71]. Another aspect is related to the global loss of nitrogen (up to 70%) due to urease activity if urea is applied as a fertilizer. Therefore, urease inhibition is one of the strategies worldwide to maintain soil fertility [72]. In our experiments, combination of dry SS with nitrogen-containing fertilizer resulted in inhibition of urease activity in loamy soil during the vegetation experiment with maize [73].

The addition of SS-derived organics to soil increases the Cmic/Ctotal and Nmic/Ntotal ratios in the soil. At the same time, application of SS containing heavy metals, according to Fließbach et al. [8] and Chander and Brookes [74], Cmic/Ctotal ratio decreases to 32% and 50%, respectively. This effect can be developed greater in sandy soil than in clayey soils [75].

#### **6.3 Indicators of microbiological contamination**

In the early nineteenth century, the total coliforms, fecal coliforms, and fecal streptococci were considered as typical indicator bacteria. Later it was shown that these pathogens are not a major concern in solid waste landfills or leachate [62, 76]. Nowadays, different types of bacteria (fecal coliforms and *Escherichia coli*, *Salmonella*, Shigella, *Vibrio cholerae*); diverse parasite cysts and eggs (*Balantidium coli, Entamoeba* 

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

*histolytica*, and *Giardia lamblia*, helminths); viruses (human adenoviruses, enteroviruses (e.g., polioviruses), diarrhea-causing viruses (e.g., rotavirus), hepatitis-A virus and reoviruses) and fungi are monitored as biological contaminants of SS. Depending on the type and amount, they can all be harmful to the environment and human health [62, 76].

### **7. Effect of sludge-derived fertilizers on the plant growth**

Soil amendment with SS is useful for enhancing crop production, as well as the accumulation of nutrients and organic matter in the soil. However, the accumulation of humic substances (HS) in soil and plant tissues must be regularly observed in case the SS is continuously used [14]. The SS can be used as fertilizers also after pyrolysis [77]. Both sole application of SS and their respective biochars provided enough P for the plants to achieve biomass higher than conventional P-fertilizer [77].

The effect of SS on plant growth differs depending on the SS application method, that is, at the soil surface "mulching" or mixed homogeneously with soil. The application of SS on the surface has some advantages, that is, water evaporation is limited by forming a physical barrier that allows soil moisture to be retained longer. Due to those, the biological and chemical processes of organic matter transformation intensified [78]. For example, the best yield of wheat (*Triticum durum* Desf.) was obtained when SS (dried) is applied at the clayey-silty soil surface (mulching) as compared to homogeneously mixed SS with soil [78]. Plant response to SS in dependence on SS application rate, plant species, soil type, and experiment conditions is shown in **Table 1.**

Importantly, a direct application of SS on agricultural soils is not recommended. It was shown that the hygienically treated (by liming) SS inhibited the growth of white mustard (*Sinapis alba* L.) already at a ratio of 10%. The addition of compost (5%, 15%, and 25%) resulted in the suppressed phytotoxicity of sludge in all tested ratios, that is, from 5 to 50% [92].

Our experiments showed that the use of SS affects the germination and development of seedlings. Concentrations exceeding 7 g kg−1 inhibited the germination of cucumber seeds and resulted in necrotizing primary roots. In the study with airdried SS mixed with agricultural sandy loam soil at rates of 0 (control), 10, 20, 30, 40, and 50 g kg−1 (equal to 0, 30, 60, 90, 120, and 150 t ha−1), seed germination of broad beans (*Faba sativa* Bernh.) decreased from 70.0% (control), to 63.3, 56.7, 50.0, 50.0 and 46.7%, respectively [14]. Nevertheless, all the growth and morphometric parameters of broad beans positively respond to SS-amended soil compared to nonamended soil. The most effective for biomass yield of broad beans was the application of 120 t ha−1 SS [14]. In experiments with barley, the stimulation effect of SS also was shown, particularly, the addition of SS 40 g kg−1 soil led to an increase of dry weight, leaf area, number of leaves, and tillers per plant [18].

Our recent study demonstrated a positive effect of SS on maize growth and soil microbiological activity, when SS is applied in combination with mineral fertilizers [73]. Additional experiments have been performed also with cucumbers and leaf mustard. The SS preparation alone did not provide the plants with mineral nutrients in appropriate values, while the combination of SS preparation with nitrogen-containing fertilizers significantly improved the plant growth and promoted plant development [73] (**Figure 6**). This may have a long-term favorable effect on plant mineral nutrition. Our data also showed that different plants respond to the SS differently.


#### **Table 1.**

*Plant growth in response to the presence of SS in soil.*

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

#### **Figure 6.**

*The effect of SSP on the growth of plants: A—cucumbers, B—leaf mustard, C–E—maize. Label color: pink—SSP + NPK, orange—NPK, blue—SSP + PK, green—SSP, yellow—vermicompost, white—soil without fertilizers. SSP—sewage sludge preparation; PK—phosphorus and potassium-containing fertilizer; NPK nitrogen, phosphorus, and potassium-containing fertilizer. Controls—loamy soil without additional fertilizer, soil mixed with mineral fertilizer (Kristalon 18:18:18). Period of vegetation experiment A—18 days, B—47 days, C—33 days, E—46 days, and D—62 days. The application rate of SSP is 17.3 g L−1 in a loamy soil. Methods are according to Dubova et al. [73].*

A species-specific effect, in that case, can be explained by (i) different sensitivity of plants to the compounds in SS preparations; (ii) demand for mineral elements at the early stages of ontogenesis due to slow release of nutrients from SS; (iii) insufficient maturing and the presence of growth inhibitors in SS.

### **8. Environmental impact of the sewage sludge**

Sludge production globally in 2017 was 45 MT by dry matter, and now it is increasing annually due to urbanization and population growth [34, 93]. In this respect, the environmental impact of SS in the case of landfill disposal, agricultural use, or other applications is of great importance. Particularly, the contribution of different processes of SS treatment for agricultural use is recently studied by [59]. Energy consumption for SS treatment contributed mostly to global warming (>50%), while SS transportation to agricultural areas affected terrestrial and freshwater ecotoxicity, as well as ozone formation—terrestrial ecosystems (**Figure 7A and B**). Sludge disposal in agricultural areas mostly contributed to human toxicity, terrestrial acidification, and freshwater ecotoxicity (**Figure 7C**). The main impacts of SS in soil are related to the presence of Zn, which affects freshwater ecotoxicity and human toxicity [94].

Biogeochemical emissions from SS handling and spreading on land are expected to be minimized in the future by efficient utilization of nutrients and other resources derived from SS, according to the principles of a circular economy [95, 96].

#### **Figure 7.**

*Environmental life-cycle assessment of the sewage treatment plant: contribution of different activities. A—energy consumption; B—transport of sludge to agricultural areas; C—agricultural areas sludge disposal. By Do Amaral et al. [94].*

The processed land-applied SS can emit volatile chemicals and gases that may act alone or in combination with one another to produce the kinds of symptoms [63].

The composition of the sludge and the concentration of pollutants in it predetermine the possibilities of its use. The presence of heavy metals, organic pollutants, and/or pathogens are the main issues associated with the reuse of SS or biosolids extracted from it. According to Manzetti and van der Spoel [97], the following aspects can be reported—(a) raising of the levels of persistent toxins in soil, vegetation, and wildlife, (b) potentially slow and long-termed biodiversity reduction through the fertilizing nutrient pollution operating on the vegetation, (c) greenhouse gas emissions, and (d) the release of odorous compounds. Groundwater contamination from biosolids with pathogenic microorganisms is one of the greatest problems worldwide, due to the lack of adequate and equitable sanitation of SS [98]. Chemical contaminants in processed SS may potentially interact with microbial pathogens, thus, causing or facilitating the disease process via allergic and nonallergic mechanisms, as well as microbial byproducts [63]. Furthermore, endotoxins and exotoxins, which are produced by most bacteria in SS and retain their toxicity at extremely high dilutions, can cause severe illness or death. Endotoxins are heat stable even upon autoclaving, while can be inactivated with dry heat at temperature above 200°C for 1 h [79, 99, 100]. A high microbial diversity of SS leads to the horizontal gene transfer and proliferation of antimicrobial resistance (AMR) [101]. The virus persistence in SS is dependent on the physicochemical and biological properties. For example, enveloped viruses survive for 6–7 days in SS [102], while SARS-CoV-2 might persist on the surfaces up to 72 h [69]. Coronavirus can persist in domestic and hospital SS also for a longer period of time at lower temperatures (4°C) [62, 103].

Long-term accumulation of toxic elements in soil and their uptake by plants is currently the biggest concern in terms of direct SS land application. The bioavailability of heavy metals in the soil is closely related to the value of the soil exchange reaction (soil pH measured in KCl or CaCl2 form), as well as to the sorption properties of the soil, which change with the addition of SS. According to published data, the availability of heavy metals in soils decreases in the order (Zn + Cd) > (Ni + Cu) > (Pb + Cr). However, in connection with physicochemical processes, the accumulation of heavy metals may occur over time, so it is necessary to monitor their concentration for a long time after the application of sludge [104]. When sludge is incorporated into the soil, the heavy metals in it bind to organic matter and clay particles, which usually accumulate in the soil [8, 105].

In Latvia, no more than 14 t ha−1 of dry matter may be incorporated at a time with sludge or compost. This corresponds to 55 t ha−1 of naturally moist sludge with a dry matter content of about 25% [106]. For 18 years, the concentration of heavy metals in Jelgava SS has significantly decreased. Similar trends have been observed in other treatment plants and this shows that heavy metals are no longer the most important limiting factor for the use of SS.

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

Wastewater can transport plastics from many different sources, such as fibers from washing machines, personal care products, and facial scrubs. WWTP efficiently removes the microplastics (MPs) from the wastewater, essentially trapping the particles in the sludge [107, 108]. Studies of Peterson [76] showed that 9 years of repeated sludge application led to the accumulation of MPs in the soil. According to various studies, MPs pose various negative effects on soil ecosystems, such as affecting soil fertility, soil organisms' fitness, soil texture, and decreasing crop yield [109, 110].

Pignattelli et al. [111] highlighted the toxicity caused by small MPs (PP, PE, and PVC) on the growth of garden cress (*Lepidium sativum*). Hernández-Arenas et al. [112] studied the effect of MPs in sludge on the growth of tomato plants and discovered that plants grown in soils treated with sludge with a high concentration of MPs had the lowest biomass and did not produce any fruits during the experiment.

Domestic SS is a major source of pharmaceuticals, drugs, and antibiotic resistance genes, so it is important to ensure its biodegradation during sludge treatment. Drugs can remain in the sludge even after stabilization (dewatering), due to their high sorption capacity [113]. Ivanová et al. [114] discovered more than 100 types of drugs and their metabolites in SS. The amount and type of antibiotics in wastewater affect also the composition of bacteria [115].

Pharmaceutical substances are subject to thermal decomposition over a wide temperature range; therefore, it is possible to expect a reduction in the content or their complete removal during thermal processes [116]. Szabová et al. [117] achieved almost 100% drug removal in the sludge by heat treatment at 250°C and incineration at 550°C. Furthermore, pyrolysis at 350–500°C is able to decrease the concentration of MPs in sludge by more than 99% [118].
