1 Introduction

Increased efficiency of municipal waste water treatment systems results in the generation of greater amounts of sewage sludge. Sewage sludge management is an important issue in any modern municipal waste water treatment plant. Based on demographic forecasts, it is estimated that the amount of sewage sludge that will be produced in Poland in 2018 will increase from 612,800 to 706,600 Mg of dry matter in 2010 [1]. On-site storage and temporary warehousing of sewage sludge in treatment plants and its use for land reclamation, including for agricultural purposes, are less and less popular, while alternative methods of its transformation are sought. Methods of thermal transformation of sewage sludge starts to dominate both in Poland and in the world [2,3,4].

The type of sewage delivered to waste water treatment plants and the method of its treatment determine the physical and chemical properties of sewage sludge. The chemical composition of sewage sludge depends on the chemical composition of sewage sludge can contain a large variety of elements and compounds, including contaminants, such as heavy metals and dioxins, furans, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, as well as halogen derivative compounds. Energy properties of sewage sludge also depend on the type of sewage delivered to the treatment plant and on the process of sludge treatment [5]. The most important properties determining the energy potential of sewage sludge are: the content of dry matter, volatile matter, mineral substances, heat of combustion and calorific value; the elemental and chemical compositions of ashes are also of great importance.

The dry matter content or—in other words—hydration of sewage sludge appears to be a key parameter of sewage sludge hindering the energy use of this type of materials. That is why, in western European countries, such as Belgium, the Netherlands and Austria, sewage sludge is co-combusted with hard coal [6]. Mixing sewage sludge with plant waste is another approach and can also significantly reduce the hydration of these materials and improve the physical characteristics in the combustion process, which will encourage better energy efficiency [7]. Mixing sewage sludge with plant materials may also bring significant changes in leaching of, among others, heavy metals which, when released from organic compounds and accumulated in ashes, may pose a potential threat to individual links of the food chain [8]. According to Hong et al. [9] sewage sludge combustion can have a significant impact on the global warming potential (GWP).

The aim of this study was to evaluate the influence of plant biomass added to sewage sludge on the product energy potential.

2 Experimental Section

2.1 Material Sampling, Characterization, and Pretreatments

Sewage sludge used in the study came from municipal waste water treatment plant located in Krakow (southern Poland). The sewage sludge was stabilized before sampling. After initial concentration, the sludge was subjected to mechanical disintegration (cavitation at a sudden reduction of pressure), then it was redirected to biological reactors or to an intermediate tank. At this point, the sludge was put into separate fermentation chambers in which it was mixed with a stirrer and heated on heat exchangers. The sludge was fermented in separate chambers for 19 days. After fermentation, it was de-watered on a belt press.

Materials used as components for the preparation of sewage sludge mixtures were selected taking into account the ease of access as well as the efficiency of improving the physical properties of sewage sludge. Wheat straw used in the study came from a farmstead. Sawdust and bark of conifers were obtained as sawmill waste. Elemental composition of sewage sludge and waste plant materials is presented in Table 1.

Table 1 The content of dry matter and N, C, H, S, O in plant materials and sewage sludge used in the experiment
Full size table

In order to improve its physical properties, sewage sludge was mixed with plant materials. Before mixing with sewage sludge, wheat straw (WS) and bark of conifers (B) were shredded and 5 mm sieved. Sawdust of conifers (S) was used in a form in which it was obtained. Materials with a natural water content were mixed at 1:1 weight ratio on a dry matter basis. The scheme of the experiment was as follows: sewage sludge without any additions (SS), sewage sludge + wheat straw (SS + WS), sewage sludge + sawdust (SS + S), sewage sludge + bark (SS + B).

The characteristics of the chemical composition of plant feedstocks, sewage sludge and mixtures used in the study were presented in the article of Gondek et al. [10].

The materials were analyzed in order to evaluate the influence of plant biomass added to sewage sludge on the product energy potential. The energy potential of feedstocks and mixtures was studied after determining their elemental composition, moisture, the contents of volatile matter and combined carbon, heat of combustion and calorific value. Dry matter content in the materials was determined at 105 °C for 12 h [11]. Elemental composition (C, H, N, S) was determined in the samples using a CHNS Vario EL Cube analyzer manufactured by Elementar. Total O was derived by subtraction according to DIN 51733 method as follows:

$$ {\text{O}}\;\left( {\% {\text{w}}/{\text{w}}} \right) = 100{-}{\text{ash}}\;\left( {\% {\text{w}}/{\text{w}}} \right) - {\text{C}}\;\left( {\% {\text{w}}/{\text{w}}} \right){-}{\text{N}}\;\left( {\% {\text{w}}/{\text{w}}} \right){-}{\text{H}}\;\left( {\% {\text{w}}/{\text{w}}} \right){-}{\text{S}}\;\left( {\% {\text{w}}/{\text{w}}} \right) $$

The ash content was determined according to PN-EN 14775:2010 method by calculation from the mass of the residue remaining after the sample was heated in air under rigidly controlled conditions of time, sample weight and equipment specifications to a controlled temperature of (550 ± 10) °C.

The volatile content was determined according to PN-EN 15148:2010 method. A test portion of the general analysis sample is heated out of contact with ambient air at (900 ± 10) °C for 7 min. The percentage of volatile matter is calculated from the loss in mass of the test portion after deducting the loss in mass due to moisture.

The calorific value was determined according to PN-EN 14918:2010 method. The above determination was performed on a sample with a mass of 1 ± 0.1 g placed in a bomb calorimeter in the form of a compressed pellet. Ignition of the sample was made using a cotton thread with a diameter of 0.1 mm embedded into the pellet. The calorific value was calculated by a computer program controlling the operation of the calorimeter.

3 Results and Discussion

3.1 Chemical Composition of Materials

Sewage sludge used in the study had a relatively low dry matter content compared to wheat straw, sawdust, and bark of conifers from which the mixtures were prepared. The addition of plant materials to sewage sludge was an important factor influencing the reduction of water content in sewage sludge [12] (Table 1).

Elemental composition of materials used in the study differed significantly (Table 1). The highest nitrogen and sulfur contents were found in sewage sludge. The addition of plant materials diluted both components in the mixtures. Different results were obtained for carbon in which case the addition of plant materials led to a significant increase in the C content in the mixtures compared to the content of this element in sewage sludge itself. Although the hydrogen content in plant materials was higher than in sewage sludge, no increase in the element content in the mixtures was noted. The oxygen content in sewage sludge and plant materials was varied. However, it should be noted that the addition of plant materials resulted in an increase of this element in the mixtures compared to sewage sludge.

3.2 Energy Potential of the Materials Used in the Study and of the Mixtures Prepared

The study showed that the addition of plant biomass to sewage sludge reduced the moisture content in the mixtures (Table 2). Compared to sewage sludge without any additions, the highest reduction of moisture content was determined in the mixture of sewage sludge and bark (SS + B). This is important information for evaluating the amount of energy needed to drain water from the fuel, the calorific value, and the content of ash generated [13]. As stated by Rulkens [14], the energy efficiency of sewage sludge combustion is strongly influenced by the level of dehydration of such materials.

Table 2 Energy potential of plant biomass, sewage sludge, and mixtures
Full size table

The ash content was the highest in bark and it was due to the presence of mineral contaminants in the material during the woodworking process (Table 2). This also resulted in the highest content of this component in the mixture of sewage sludge and bark. Compared to other mixtures, the component content was higher by more than 50%, which could significantly affect the content of ash generated (fuel ballast) and its further disposal, and also significantly deteriorate the combustion process and raise the cost of energy production.

Volatile matter is one of the most important components of the fuel that determine the combustion process. The greater the content of volatiles in the fuel, the easier the ignition and the faster the combustion. Insufficient content of volatile matter results in the loss of stability of the combustion process. The addition of plant biomass to sewage sludge in the study increased the content of volatiles in the mixtures (Table 2). Compared to sewage sludge without any additions, the highest increase of volatile content was determined in the mixture of sewage sludge and sawdust. However, it should be noted that this was a lower value compared to that in the sawdust itself.

The addition of plant materials to sewage sludge increased the content of fixed carbon (Table 2). The highest fixed carbon content was found in the mixture of sewage sludge and bark. Regardless of the mixture, the fixed carbon content was lower compared to that determined in plant materials used in the preparation of the mixtures.

The addition of plant biomass to sewage sludge had a significant effect on the heat of combustion in the analytical state of the mixtures prepared (Table 2). The highest heat of combustion value was observed for the mixture of sewage sludge and sawdust, i.e. the material with the lowest ash content and the highest volatile content among the mixtures prepared. However, it should be emphasized that although the heat of combustion of the mixtures prepared was higher than in the case of sewage sludge without any additions, the heat of combustion values were lower than these determined for wheat straw and sawdust. Also Manara and Zabaniotou [15] found that mixing sewage sludge with biomass improves fuel properties and increases the efficiency of the process. In addition, mixing sewage sludge with biomass dilutes toxic components.

The calorific values of plant biomass, sewage sludge and mixtures followed a similar trend compared to the heat of combustion. According to Bożym [13], the calorific value of sewage sludge is derived from the chemical composition and presence of volatiles. The addition of plant biomass to sewage sludge increased the organic matter content, resulting in an increase in the calorific value.

4 Conclusions

The addition of plant biomass to sewage sludge reduced the moisture content and increased the volatile matter content compared to sewage sludge without such addition, in which case the variation was relatively low. For the studied mixtures, larger variations were observed in relation to the ash and combined carbon contents. The highest contents of ash and combined carbon were found in the mixture of sewage sludge and bark. The highest value of heat of combustion was determined in the mixture of sewage sludge and sawdust (14,000 J g−1). Calorific values of the mixture of sewage sludge and wheat straw and sewage sludge and bark were 13,640 and 11,540 J g−1, respectively, and were higher by more than 40% on the average compared to the calorific value of sewage sludge without any additions.