Analysis of Sewage Sludge Disposal Routes – Varazdin and Medimurje County

: The aim of this paper is to analyse sewage sludge production at the level of Medimurje and Varazdin counties with economic analysis of acceptable technical/technological solutions for sludge management routes. The topic is very broad, so the focus was placed on four variants of sludge disposal, which were determined by preliminary analysis to be the most applicable for the area in question. These are: 1) disposal of sewage sludge in agriculture, 2) incineration of sludge in a central mono-incinerator, 3) composting for the area of Varazdin County and at the same time use of sludge reed beds for the area of Medimurje County, and 4) export of sludge outside the state borders. Based on the analysis carried out for the described four variants, sludge incineration in a regional mono-incinerator turned out as the economically most advantageous solution, while the variant that includes disposal of sludge in agriculture has showed as the most expensive one. The last one also showed the largest range of costs uncertainties, but the mentioned facts are greatly influenced by unfavourable legislation, the changes of which in a more rational direction would greatly contribute to the greater competitiveness of this variant. Overall, taking into account the lowest cost and uncertainties in the predicted unit costs, the sludge composting in the Varazdin County and the construction of a reed beds for the Medimurje County, turned out to be the optimal variant. This solution is also the one that was ultimately adopted in practice.


INTRODUCTION
The wastewater collected by the public drainage system is taken to the wastewater treatment plant (WWTP) and after reaching the required level of treatment, it is discharged into the recipient.The by-product of every technological process of municipal WWT is sewage sludge, which requires further processing and disposal in the most economical way, while considering its technical and ecological suitability.On average globally, generated unit quantities of daily sludge production amount to 35 -85 g DM/PE (dry matter / population equivalent) [1].The available data for Croatia, based on operational WWTP, show, this value amounts to 50 -55 g DM/PE daily [2].
The aim of this paper is to analyze the sludge production for Medimurje and Varazdin counties and give analysis of economically acceptable technical and technological solutions for sludge treatment and disposal routes.
All conventional sludge treatment methods ultimately generate stabilized and dehydrated sludge that also requires further disposal.Also, additional sludge processing procedures result in either a new form of sludge or a new byproduct of its processing: dried sludge, sewage sludge ash, compost (or sludge like compost within reed beds) etc.So, all these processes generate sludge, ash or compost that ultimately needs to be disposed of somewhere or used in some other applications [4].
According to EUROSTAT data, even at the EU level there are no unified strategies or guidelines for sludge disposal.Use in agriculture is dominant method in Ireland, Portugal, UK, Denmark, Spain and Luxembourg, while in the Baltic countries, as well as Finland, Slovakia, Hungary and Czech Republic, the use of sludge on non-agricultural lands prevails.On the other hand, in some of the most developed countries such as the Netherlands, Switzerland, Germany, Belgium and Austria, thermal treatment of sludge, primarily incineration, is the predominant method of its disposal.Although limited and for the most part even prohibited by EU regulations, disposal of sludge in landfills is still the predominant method of disposal of sludge in Italy, Greece, Romania, but unfortunately also in Croatia [5].
According to Tarpani et al. [6] over 70% of generated sludge in Europe is treated thermally (incineration) or used as a fertilizer in agriculture.The best solution is to include the waste produced from wastewater treatment into the cycle of matter and energy, preferably as it happens in nature, but also in any other sustainable way.This is also the basis of Council Directive 91/271/EEC [7] which states: "Sludge from wastewater treatment will be reused whenever possible." Since there are many acceptable methods of sludge treatment (thickening, stabilization, dehydration, thermal hydrolysis, drying, solidification, mono-incineration, pyrolysis, gasification, composting, grounding, etc.), combined with many acceptable methods of its final disposal and/or use (landfilling treated sludge, use of sludge in agriculture or on non-agricultural land, disposal of ash in specially arranged landfills for non-hazardous waste, use of sludge (ash) in construction, different types of coincineration of sludge (e.g. with municipal solid waste or in cement kilns), export of dehydrated or dried sludge or sewage sludge ash outside the country in which it originated), the resulting number of different solutions for complete sludge management is even higher, which further complicates the process of finding the optimal solution.To select the optimal solution (or combination of solutions), it is usually necessary to apply a multi-criteria decision methodology, which involves defining the criterion structure and evaluating the variants according to the adopted criteria [8].In the continuation of this paper, a technical-economic analysis of the selection of the optimal variant of sludge disposal for area of northern Croatia will be presented.Nevertheless, it is noted that within the framework of conducting a comprehensive analysis, as part of future steps, it is necessary to conduct an analysis of environmental and sociological impacts, which would complete the whole in terms of the basic requirements of sustainable development.
The relevant input data for determining sewage sludge production was the projected capacity of the agglomerations (WWTPs) in the Medimurje and Varazdin counties defined as part of the Water Area Management Plan 2016-2021 [3] and corrected according to data obtained from the responsible utility companies Medimurske vode d.o.o. and Varkom d.o.o.
The analysis covers 7 agglomerations of more than 2,000 population equivalent (PE) in Medimurje County and 11 agglomerations in Varazdin County, as shown in Fig. 1.

COST ANALYSIS
For the considered WWTPs, the amounts of generated sewage sludge were calculated, as well as the production of dehydrated sludge with 23 and 33% DM and the amount of generated dry sludge in the case of solar drying up to 75% DM (Tab.1).Ash production (t/year) was also calculated in the case that the entire sludge is incinerated in a central mono-incinerator presumably located next to WWTP Varazdin, as by far the largest one within analyzed area.The starting point for determining sludge production was the predicted capacity of built and planned WWTPs.The capacities of the WWTPs considered were taken over as the planned capacities of the planned agglomerations from the Water Area Management Plan 2016-2021 year, corrected according to data obtained from the responsible utility companies Medimurske vode d.o.o. and Varkom d.o.o.Data on the unit costs of solar drying, transportation of dehydrated and dried sludge, sampling and analysis of sludge, transaction costs and premiums to farmers for disposal in agriculture, data on the costs of composting and reed beds, unit costs of sludge incineration in mono-incinerators and ash disposal, as well as the costs of sewage sludge export outside the country, were taken from the Action Plan for Sewage Sludge Disposal [9] and Vouk et. al. (2017) [10] and corrected to a certain extent considering the current situation and price movements on the market.
In the particular case, already by carrying out preliminary analyses, the most cost-effective solution was to build plants for sewage sludge solar drying in two locations, the largest WWTPs for each county.The location of WWTP Cakovec was chosen as the location of the central plant for solar drying of sludge in Medimurje County, and the location of WWTP Varazdin as the location of the central plant for solar drying of sludge in Varazdin County, considering that such an approach results in a lower unit price of total sludge treatment (solar drying including transportation costs).The calculated costs of sludge solar drying include the cost of building, operation, maintenance and depreciation of solar sludge drying plants and the cost of road transport of dehydrated sludge from individual WWTPs to the regional center in Cakovec or Varazdin, as shown in Tab. 2.  The variant that includes composting of sludge in the Varazdin County and reed beds in the Medimurje County involves the construction of a composting plant in Varazdin and the transportation of sewage sludge from smaller WWTPs in the Varazdin County to the central composting plant and the construction of reed beds in Cakovec and the transportation of sludge from smaller WWTPs in the Medimurje County on the reed beds located by the Cakovec WWTP.The total cost includes the construction, operation, maintenance and depreciation of the composting plant in Varazdin and the cost of road transportation of dried sludge from smaller WWTPs to the composting plant, as well as the cost of reed beds construction, emptying and sludge removal, sampling and the cost of road transportation of sludge from smaller WWTPs to the reed beds (Tab.3).
In practice, the solution with the construction of reed beds in Cakovec and a composting plant in Varazdin was adopted.This includes collecting of sludge from smaller WWTPs in the area of Medimurje and Varazdin County, on two biggest central WWTPs, but it does not exclude the possibility that some of the smaller WWTPs will adopt a separate solution for sludge disposal.Sludge that is brought from smaller WWTPs to the reed beds in Cakovec is previously thickened and stabilized and contains around 3% DM in order to ensure a good distribution of liquid (nondehydrated) sludge over the entire surface of the reed beds.The variant with disposal of sludge in agriculture includes the cost of transporting dried sludge to agricultural areas without any restrictions, located within a radius of up to 50 km from any of the considered WWTP assumed following unit costs: the cost of 13 €/t for transportation within a radius of 10-50 km, the cost of sampling and analysis (for this analysis was assumed to be 90 €/t of dried sludge), transaction costs (5 €/t of dried sludge) and premium for farmers which take over sludge (10 €/t of dried sludge), This is shown in detail in Tab. 4.
The variant with sludge incineration in a monoincinerator includes the construction of a mono-incinerator within the WWTP Varazdin perimeter and the collection of dried sewage sludge from two central plants for sludge solar drying: in Cakovec and Varazdin.The central plant for solar drying in Cakovec is 21 km distant from the location of the mono-incineration plant, while the plant for solar drying in Varazdin is within the WWTP perimeter and in the immediate vicinity of the incinerator.The cost of incineration in a mono-incinerator includes the cost of solar drying, the cost of transporting sludge from central plants for sludge solar drying to the mono-incinerator, the cost of construction, operation, maintenance and depreciation of the monoincinerator (with assumed incineration unit price at €65/t DM) and the cost of ash disposal (Tab.5).It was assumed that the ash will be disposed of at an organized nonhazardous waste disposal site, close to the incinerator.The solution that envisaged sewage sludge export outside the state assumes that a certain legal entity (company) will take over dried sludge (with around 75% DM) from the planned two locations of the sludge solar sludge plants and transport it over the border to Hungary where it will be disposed.In this variant, the cost of sludge solar drying and the cost of payment to a legal entity, which includes the cost of transportation and disposal of sludge outside the state, are included in total costs (Tab.6).Given that the transportation represents the most significant component of the final price, solutions with the export of dehydrated sludge that excludes sludge solar drying (sludge with 22% and 33% DM) are not economically acceptable, since significantly larger total quantities of sludge would require transportation over significant distances.This would not only inevitably result in unreasonably high costs but would also have a negative impact on the environment (increased emissions from transport, etc.), ultimately resulting in the reduced overall sustainability of such a solution.It is noted here that the final method of disposal in the country to which the sludge is exported (in this case Hungary) is left to the choice of the legal entity (company) that took over the sludge and charged an adequate fee included in the presented analysis (assumed with the amount of 105 €/t as shown in Tab. 6).However, given that in this particular case receiving country is Hungary and taking into account the currently prevailing method of sludge disposal, it is to be expected that this sludge, in a certain way, would ultimately be disposed of in agriculture.
Based on the analyzes carried out for the described four variants (Fig. 2) in the area of Medimurje and Varazdin County, it is evident that sludge incineration in a regional mono-incinerator represents economically the most advantageous solution with the total cost estimated of €1.18 million/year.In the second place, slightly more expensive, is the variant with reed beds in the area of Medimurje County and composting of sludge in the area of Varazdin County, 7.81% more expensive than the variant with sludge incineration.Although the variant with disposal of sludge in agriculture turns out to be economically the most unfavorable (47.97% more expensive than the variant with incineration), with the adoption of more rational legislation, i.e. by reducing the costs of sampling and analysis of sludge and soil, its better economic profitability could be expected.Export of sludge outside Croatia, although a more costeffective variant than the use of processed sludge on the agricultural land (16.76% more expensive than the variant with sludge incineration), does not represent an acceptable solution.It is justified to use it only as a temporary solution until the establishment of an appropriate sludge management system in the region in question.Considering that all the presented results and rankings of individual variants are based on the assumed unit prices of individual processes, a kind of sensitivity analysis was carried out bellow.

RISK ANALYSIS
Given that costs in sludge disposal analysis are variable, and of stochastic character, it is necessary to carry out additional risk analysis that determines dependence of the final results on changes in assumed unit prices.Hereby, the ranges of possible total costs for each of analysed variants of sludge disposal were defined (Tab.7).
Below are the results of the economic risk analysis, giving an indication of the possible ranges of total costs of the considered scenarios relation to the assumed fixed and variable inputs (unit costs).In the concrete example, the unit costs of the following parameters were varied: sludge solar drying total cost, the cost of sampling and analysis of sludge and soil (when it is used in agriculture), premium for farmers cost, total incineration costs, ash disposal costs, total cost of sludge export outside the country, total costs of reed beds and total composting costs were selected as stochastic parameters.Tab.7 shows the analyzed ranges of variable parameters considered through risk analysis.In the same way, it is possible to include other and additional parameters through risk analysis, e.g. the composition of the sludge generated, the capacities of selected WWTP, etc.The following values are defined for all the selected stochastic inputs: minimum, maximum and most likely value.Specific values were determined on the basis of collected literature data and actual data characteristic of the analyzed geographical area.Fig. 3 shows the expected (possible) ranges of total costs for each of the four analyzed variants of sludge disposal.Regarding the presented results of the performed risk analysis, the alternative where sludge is being composted in Varazdin County and sludge reed beds are used in Cakovec County proved to be the most acceptable.This solution is also the one that was ultimately adopted in practice (Fig. 4 and Fig. 5).At the same time solution with agricultural use of sludge shows the largest range of possible expected costs, and thus risks.However, with the reduction of specific sampling and analysis costs when sludge is used in agriculture, a significant improvement in the profitability of this solution is to be expected.Incineration of sludge in central mono-incinerator located by the WWTP Varazdin, although the most favourable if only the most probable (expected) value is observed, shows somewhat larger deviations in the positive direction (possible increase of the total cost), which is why this variant is ultimately ranked behind the selected solution with composting and sludge reed beds.
However, it is necessary to emphasize that all analyses and evaluation of individual options refer exclusively to economic criteria.In reality, it is necessary to include other criteria, primarily environmental impacts, which in such cases are most often quantified using life cycle analysis (LCA) and models based on them [10].With the eventual inclusion of additional criteria, such as sociological ones, the analysis can be extended by carrying out a multi-criteria analysis.Also, the sensitivity analysis can be expanded to additional parameters by varying the expected amount of sludge generated at each WWTP or its basic characteristics (such as moisture content) etc.

CONCLUSION
Different sewage sludge disposal routes in the area of northern Croatia (Varazdin and Medimurje County) were analysed regarding technical and financial considerations.Four variants (solutions of sludge disposal) were considered: sludge incineration in central mono-incinerator, use of sewage sludge in agriculture, combination of reed beds and composting in two regional centres and export of the sludge outside the state.
The total capacity of all WWTPs (agglomerations) considered is 337.756PE, i.e. 150.723PE in the area of Medimurje County and 187.033PE in the area of Varazdin County.Results show that the economically most favourable is to build 2 plants for sludge solar drying: by the WWTP Cakovec -central plant for Medimurje County, and by the WWTP Varazdin -central plant Varazdin County.Based on the analysis carried out, it is evident that sludge incineration in a regional mono-incinerator represents the economically most advantageous solution, at the total cost of €1.18 million/year.In second place is the variant with reed beds in the area of Medimurje County and composting of sludge in the area of Varazdin County, 7.81% more expensive than the variant with sludge incineration.
Even though the variant with sludge disposal in agriculture turned out to be the most economically unfavourable (as much as 47.97% more expensive than the variant with incineration), with the adoption of more rational legislation, i.e. by lowering the costs of sampling and analysis of sludge and soil, better economic profitability can be expected.Export of sludge outside of Croatia, does not represent an acceptable solution, and could be justified only as a temporary solution until the establishment of an appropriate sewage sludge management system in the region.Based on the results of the economic risk analysis, a solution with combination of reed beds in Medimurje County and composting in Varazdin County turned out to be the most favourable regarding reliability of the analysis made and the obtained estimates of the total costs.Also, this is the solution that for the most part corresponds to the solution adopted in practice.

Figure 2
Figure 2 Total costs of sludge disposal according to the variants considered in Medimurje and Varazdin counties

Figure 3
Figure 3 Total costs of sludge disposal according to the variants considered in Medimurje and Varazdin counties

Figure 4
Figure 4 Constructed reed beds within WWTP Cakovec

Figure 5
Figure 5 Constructed composting plant within WWTP Varazdin

Table 1
Calculation of the generated quantities of dehydrated and solar-dried sludge and the amount of ash in the case of sludge incineration

Table 2
The cost of construction and maintenance of a plant for solar drying of sewage sludge with included road transportation costs

Table 3
Total cost of sewage sludge composting in the area of Varazdin County and reed beds in the area of Medimurje County *Including construction, operation, maintenance and depreciation **Including reed beds construction, emptying, sludge removal, sampling costs

Table 4
Total cost of sludge disposal in agriculture

Table 5
Total cost of sludge incineration and ash disposal

Table 6
The total cost of export and disposal of sludge outside the state

Table 7
Ranges of unit prices of assumed variable input parameters * The cost of construction, operation and maintenance of a plant for solar drying of sludge -estimated regarding the capacity of the plant next to WWTP Varazdin (lower unit value) and WWTP Cakovec (higher unit value)