*2.2.1 Air-dried sewage sludge*

Sewage sludge was obtained from the Alcázar de San Juan WWTP (29,000 inhabitants, central Spain: 391 240 N; 31 120 W. Altitude, 644 m). This WWTP purifies wastewater by a biological activated sludge system. This wastewater is a mixed kind (70% domestic, 30% industrial) and comes from two industrial areas [29] related to winemaking and cheese-making. **Tables 1** and **2** show its chemical characterisation. Organic compounds (PAHs and PCBs), *Salmonella* sp., *E. coli* and ecotoxicity were also determined.


#### **Table 1.**

*Characteristics of the air-dried and composted sewage sludge. d.m: dry matter.*


**157**

*2.2.3 Commercial fertiliser*

*Sewage Sludge to Fertilise Durum Wheat: Effects on Crop and Soil*

In view of the analytical results (**Tables 1** and **2**), waste had pH 7.01, a high moisture content (over 80%), showed high electrical conductivity (3,087 μS cm−1) and was rich in organic matter (nearly 70%). The total N (5.95%), P (6.10%) and K (0.31%) contents had an interesting agronomic value because of their fertilising potential. The N content and C:N ratio (5.95) indicated that this waste could be

No performed analysis detected *Salmonella* sp., although *E. coli* (1.4 105

forming units, cfu) was found. According to Spanish Royal Decree 1310/90 [30], metals did not exceed the legal limits (**Table 2**). Otherwise organic compounds are limited by a European Union Directive. According to this legislation, the sum of 16 PAHs (Naphtalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene, Fluorabthene, Pyrene, Benz (a) anthracene, Chrysene, Benzo(b)fluoranthene, Benzo(k) fluoranthene, Benzo(a) pyrene, Indeno, Benzo (g.h.i.) perylene and Dibenz(a.h)anthracene) must to be below 6 mg kg−1 dry matter and the sum of the seven PCBs (PCB 28, PCB 52, PCB 101, PCB 118, PCB 138, PCB 153, PCB 180) cannot exceed 0.8 mg kg−1 dry matter. The content of PAHs and PCBs in the studied air-dried sewage sludge was less than 10 μg kg−1 dry matter and were, therefore,

As the purposes of using sewage sludge was an agriculture application, an ecotoxicity test was carried out to estimate its potential post-application risk [31]. According to Spanish law [32], waste is characterised as ecotoxic and is, therefore, dangerous when the Effective Concentration 50 (EC50) value is 3,000 mg L−1. In view of the analysis results (502,700 mg L−1), it can be stated that the Alcázar de

The sewage sludge from the Alcázar de San Juan WWTP was composted by a wind-row composting system. Cereal straw (80% dry matter: 4.2% crude protein, 73% fibre (cellulose, hemicellulose and lignin), 0.35% Ca, 0.15% and P, 2.36% K) was used as a carbon source and also for helping to improve the compost structure thanks to its fibrous texture. As the sewage sludge required being dried beforehand, it was spread over an area of the composting plant before being incorporated. The downwards to upwards order of the materials for pile formation was straw plus sludge plus straw. The composting ratio was two parts straw to one part sludge (v/v). The straw and sludge mixture were periodically flipped to homogenise it and to improve the ventilation of piles. Straw was applied without grinding, which not only provided the mixture with C, but also helped to improve the compost structure. The composting process was completed 4 months after starting to dry sewage

High values of electrical conductivity (5,740 μS cm−1) and Fe (10,301 mg per kg of dry matter) were detected. CaO (12.53%) and MgO (1.04%) concentrations were higher than that of the air-dried sewage sludge (6.78% and 0.87%, respectively). Moisture was 9.28%. The C:N ratio after composting was 6.68 (**Table 1**), so it was a fairly mineralised compost. It had high organic matter (52.08%) and total N (4.30%) contents. The concentration of metals (**Table 2**) was lower than that allowed by law. It was not ecotoxic (<2 toxicity units) and was a microbiologically stable compound.

NPK 8–16-8 + organic matter (22%) was the commercial fertiliser employed in the control subplots. This fertiliser is frequently used by farmers in the area. Further

colony-

*DOI: http://dx.doi.org/10.5772/intechopen.95896*

used for composting as a nitrogen source.

lower than the EU Directive limits.

*2.2.2 Composted sewage sludge*

San Juan WWTP sludge is not considered ecotoxic waste.

sludge. **Tables 1** and **2** provide the final composition details.

Both *Salmonella* sp. and *Escherichia coli* < 1,000 c.f.u g−1 were absent.

#### **Table 2.**

*Metal content in original soils, air-dried sewage sludge and composted sewage sludge.*

*Humic Substances*

*2.2.1 Air-dried sewage sludge*

ecotoxicity were also determined.

**2.2 Characteristics of experimental materials**

Before conducting the experiment, the physico-chemical parameters of both

Sewage sludge was obtained from the Alcázar de San Juan WWTP (29,000 inhabitants, central Spain: 391 240 N; 31 120 W. Altitude, 644 m). This WWTP purifies wastewater by a biological activated sludge system. This wastewater is a mixed kind (70% domestic, 30% industrial) and comes from two industrial areas [29] related to winemaking and cheese-making. **Tables 1** and **2** show its chemical characterisation. Organic compounds (PAHs and PCBs), *Salmonella* sp., *E. coli* and

**Agronomic Parameters Air-dried sewage sludge Composted sewage sludge**

pH 7.01 5.830 Moisture (%) 86.46 9.28 Electrical conductivity (μS cm−1) 3087 5740 Total organic matter (%) 65.25 52.08 Total N (%) 5.95 4.30 P2O5 (%) 6.10 5.47 K2O (%) 0.31 0.48 CaO (%) 6.78 12.53 MgO (%) 0.87 1.04 Fe (mg kg−1 d.m.) 50688 10301 C:N ratio 5.95 6.68

*Characteristics of the air-dried and composted sewage sludge. d.m: dry matter.*

**R.D. 1310/90 values limit for soil**

> **Soil with pH > 7**

Total Cd 0.1 <0.10 1 3.0 0.33 2.38 20 40 Total Cu <1.00 <1.00 50 210.0 189 116 1 1,75 Total Cr 15.51 2.21 100 150.0 24.6 26.3 300 400 Total Hg <0.10 <0.10 1 1.5 30.1 29.9 750 1,2 Total Ni 11.37 1.5 30 112.0 512 431 2,5 4 Total Pb 11.30 11.30 50 300.0 0.45 <0.2 16 25 Total Zn <20.00 <20 150 450.0 30.9 35.1 1 1,5

**Soil with pH < 7**

*Metal content in original soils, air-dried sewage sludge and composted sewage sludge.*

**Air-dried sewage sludge**

**Composted sewage sludge**

**R.D. 1310/90 values limit for sewage sludge**

> **Soil with pH > 7**

**Soil with pH < 7**

**Soil Quero**

air-dried and composted sewage sludge were determined (**Table 1**).

**156**

**Table 2.**

**Table 1.**

**Parameter (mg kg−1)**

**Soil Villacañas**

In view of the analytical results (**Tables 1** and **2**), waste had pH 7.01, a high moisture content (over 80%), showed high electrical conductivity (3,087 μS cm−1) and was rich in organic matter (nearly 70%). The total N (5.95%), P (6.10%) and K (0.31%) contents had an interesting agronomic value because of their fertilising potential. The N content and C:N ratio (5.95) indicated that this waste could be used for composting as a nitrogen source.

No performed analysis detected *Salmonella* sp., although *E. coli* (1.4 105 colonyforming units, cfu) was found. According to Spanish Royal Decree 1310/90 [30], metals did not exceed the legal limits (**Table 2**). Otherwise organic compounds are limited by a European Union Directive. According to this legislation, the sum of 16 PAHs (Naphtalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene, Fluorabthene, Pyrene, Benz (a) anthracene, Chrysene, Benzo(b)fluoranthene, Benzo(k) fluoranthene, Benzo(a) pyrene, Indeno, Benzo (g.h.i.) perylene and Dibenz(a.h)anthracene) must to be below 6 mg kg−1 dry matter and the sum of the seven PCBs (PCB 28, PCB 52, PCB 101, PCB 118, PCB 138, PCB 153, PCB 180) cannot exceed 0.8 mg kg−1 dry matter. The content of PAHs and PCBs in the studied air-dried sewage sludge was less than 10 μg kg−1 dry matter and were, therefore, lower than the EU Directive limits.

As the purposes of using sewage sludge was an agriculture application, an ecotoxicity test was carried out to estimate its potential post-application risk [31]. According to Spanish law [32], waste is characterised as ecotoxic and is, therefore, dangerous when the Effective Concentration 50 (EC50) value is 3,000 mg L−1. In view of the analysis results (502,700 mg L−1), it can be stated that the Alcázar de San Juan WWTP sludge is not considered ecotoxic waste.

#### *2.2.2 Composted sewage sludge*

The sewage sludge from the Alcázar de San Juan WWTP was composted by a wind-row composting system. Cereal straw (80% dry matter: 4.2% crude protein, 73% fibre (cellulose, hemicellulose and lignin), 0.35% Ca, 0.15% and P, 2.36% K) was used as a carbon source and also for helping to improve the compost structure thanks to its fibrous texture. As the sewage sludge required being dried beforehand, it was spread over an area of the composting plant before being incorporated. The downwards to upwards order of the materials for pile formation was straw plus sludge plus straw. The composting ratio was two parts straw to one part sludge (v/v). The straw and sludge mixture were periodically flipped to homogenise it and to improve the ventilation of piles. Straw was applied without grinding, which not only provided the mixture with C, but also helped to improve the compost structure. The composting process was completed 4 months after starting to dry sewage sludge. **Tables 1** and **2** provide the final composition details.

High values of electrical conductivity (5,740 μS cm−1) and Fe (10,301 mg per kg of dry matter) were detected. CaO (12.53%) and MgO (1.04%) concentrations were higher than that of the air-dried sewage sludge (6.78% and 0.87%, respectively).

Moisture was 9.28%. The C:N ratio after composting was 6.68 (**Table 1**), so it was a fairly mineralised compost. It had high organic matter (52.08%) and total N (4.30%) contents. The concentration of metals (**Table 2**) was lower than that allowed by law. It was not ecotoxic (<2 toxicity units) and was a microbiologically stable compound. Both *Salmonella* sp. and *Escherichia coli* < 1,000 c.f.u g−1 were absent.

#### *2.2.3 Commercial fertiliser*

NPK 8–16-8 + organic matter (22%) was the commercial fertiliser employed in the control subplots. This fertiliser is frequently used by farmers in the area. Further details from the label are: P2O5 (soluble in neutral ammonium citrate and water), 14.50%; P2O5 (soluble in water), 11.50%; K2O (soluble in water), 8.00%; Organic C, 12.50%; Humic acids, 1.00%; Cu, 208 mg kg−1 dry matter; Zn, 559 mg kg−1 dry matter.
