Sustainable Treatment of Waste Sludge from Municipal Wastewater and its Agriculture Use

Biljana Jordanoska Šiškoska1, Valentina Pelivanoska1, Momčula Jordanoski2, Ziba Resmi3

 

1 University St. Climent Ohridski - Bitola, Scientific Tobacco Institute, Prilep, Kičevska bb, 7500 Prilep,Republic of Macedonia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

2 University St. Climent Ohridski – Bitola, Faculty of Veterinary Medicine, (retired); E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

3 MJP Proaqva, Struga, (retired)

 

Abstract

In the modern world, a special place is occupied by rational usage of the waste sludge from municipal wastewater treatment. There is always an open question for finding local solutions for the problems related to sewage sludge disposal. The aim of this paper is to sublimate the results of the research performed within the national project "Investigation of the possibility of chemical and technological treatment of sludge from the treatment of municipal wastewater treatment plants and producing complex fertilizers", which took place in two phases. The first phase of the research was laboratory and semi-industrial investigation of waste sludge from the Vranište Waste Water Treatment Plant plant near Struga. In the second phase, the field trials were set out to define the effects of the use of the produced organic mineral fertilizer. It was found that organo-mineral fertilizers derived from sewage sludge seem to be suitable for application to the soil. Therefore this work initiates multiple solutions that cover environmental effects and economic benefits from such treated product. In the absence of national legislation applicable to this problem, the European Union suppositions have been used.

Keywords: waste water, sludge, treatment plant, organic-mineral fertilizer.

Introduction

 

Waste water is a mixture of domestic wastewater (municipal sewage) and industrial wastewater. Today, it is very unlikely that only municipal or industrial wastewater will be separated. In the case of wastewater treatment systems, the biggest problem is how to bring industrial wastewater to a level that is not harmful for the process of final wastewater treatment. The obligation of industrial enterprises is, depending on the technology of work, to treat their wastewater primarily and lead them to the concentration of contaminants that will be acceptable for collector systems, which, besides municipal sewage, also accept primary purified industrial wastewater. The end product of wastewater treatment is sewage sludge. Sludge is concentrated suspension of solids, largely composed of organic matter with a range of consistencies which depend on the sludge treatment. As one of the best environmentally sustainable usages of the sewage sludge is its use in agriculture. This is strongly promoted in many countries throughout the world, first from an economical aspect, and second its usage as a secondary raw material is a valuable source of nutrients and organic matter (Circular Economy Policy-2015). On the other hand, management of sewage sludge is quite challenging whilst meeting EU legislation on pollutants, microorganisms as well as toxic compounds (The Sewage Sludge Directive 86/278/EEC, Urban Waste Water Directive, 91/271/ECC).

The wastewater treatment system in our study collects waste water from the cities of Ohrid and Struga and industrial waste water plants that are in operation. Industrial wastewater plants discharge wastewater at a satisfying quality level which is acceptable for the final recipient-collector system (Proaqva, 1988).

The aim of this research as part of the project "Investigation of the possibilities of chemical and technological treatment of sludge from the municipal wastewater treatment plants for the production of complex fertilizers" was how to obtain microbiologically sound sewage sludge and how to use this material for the production of organo-mineral fertilizers. Sewage sludge contains large amounts of organic matter and minerals, so its utilisation in agriculture appears to be a logical proposal. Municipal sewage sludge fertilizers can improve the physiochemical properties of soil and affect crop yields and growth (Antonkiewicz and Pelka, 2014; Onwudiwe et al., 2014; Baawain et al, (2014); Abdul Khaliq et al., 2017). These fertilizers are very rich in macro and micronutrients, which can supply nutrients to plants and improve their fertility (Kominko et al. 2017).

In recent years methods of sewage sludge disposal are increasing (Pająk, 2013; Kelessidis & Stasinakis, 2012; Kominko et al., 2017). In this paper we describe the method for production of organo-mineral fertilizer based on sewage sludge. This fertilizer has a balanced composition: an organic component that is always absent in soils treated with inorganic fertilizers, and mineral components, which would contain the elements necessary for vegetable crops. An important factor, which should not be overlooked, is that such produced fertilizer should not affect the composition of agricultural soil. In the next phase of the research we studied the influence of this product on field crops and its influence on arable land.

In the final phase we conducted a simplified study from setting up the investigated treatment technology for sewage sludge to the environmental effects and economic benefits of such a treated product. In doing so, European standards for municipal sewage (Directive 91/27 /ECC) and Directive 86/278 / EEC for sludge used in agriculture have been applied. Macedonian legislation on this topic, as well as regulations of neighbouring countries were also used, especially for those segments for which there are no Macedonian regulations ( Clesceri et al., 1998; TES, 2013).

 

Materials and Methods

During the preparatory period, chemical and microbiological analyses of samples of waste sludge collected after separation from the filter presses in the wastewater treatment plant in Vraniste (Struga) were made. Microbiological analyses were performed in the Hydrobiological Institute, Ohrid (Clesceri et al., 1998), while chemical analyses of the obtained product were performed at the St. Clement of Ohrid University of Bitola and the Scientific Tobacco Institute, Prilep. To provide comparative steps during the research period, field experiments were made with waste sludge that did not pass the degradation phase, as required by the technology for its treatment. Experiments were conducted on garden crops that were fertilized with the raw material. After the final filtration in the purification plant, we treated the material with lime (CaO) in the reactor. In doing so, the microbiologically correct material was obtained, which was used as a basis for the production of organo-mineral fertilizer. The complete method for the production of combined fertilizer is described in detail in Patent –P2012/417and by Vait et al. (2012, 2013).

Experiments were performed in laboratory conditions, for which an laboratory scale testing line was established consisting of: an improvised reactor with a mixing system (homogenization) where CaO (calcium oxide) is added, auxiliary bushes and a worm transport system materials. After treatment the sludge is collected in the collecting tank. In order to determine the degree of sanitation follow-up microbiological analyses were performed, Clesceri et al. (1998).

The obtained organic-mineral fertilizer, which meets the provisions in the Macedonian legislation, has been tested in an accredited laboratory. For determining total content of the elements, samples were digested on a hot-plate (ISO 14869-1). Total nitrogen, organic matter, carbonates content, extractable phosphorus and potassium, pH, Pelivanoska, (2012). According to the Regulation of the Minister of Agriculture and Rural Development, organomineral fertilizers in a solid state should contain at least 10% of organic matter, expressed as dry solids. The nutrients content cannot be less than: 1% (m/m) of total nitrogen (N), 1% (m/m) of total phosphorus expressed as phosphorus pentoxide (P2O5) and potassium oxide 1% (m/m) (K2O).

The effects of fertilization were performed on the experimental fields of the Scientific Tobacco Institute, Prilep, during the three-year research period with different fertilization rates (10, 20 and 30 t/ha) in two separate locations, Pelivanoska et.al (2015) and in the experimental field in Strimica. The experiment was performed on oriental tobacco, hot peppers (Fortes) and cornichon cucumbers.

 

Results and Discussion

The collector system and the purification plant for the protection of Lake Ohrid were designed for approximately 120,000 equivalent per capita. This provides approximately 25m3 of stabilized, concentrated sludge/day, with a concentration of 25% dry matter. This quantity from the aspect of setting anaerobic procedure for the production of methane is not economically justified, and even after the anaerobic process, solid waste remains.

 

Composition and Properties of the Raw Material

Our activities, initially, were focused on defining the composition of the waste sludge deposited in arid fields. The baseline data were the analyses given in the above-mentioned study, Vait et al. (2012). Initial analyses of the sludge, which were performed in a laboratory working within the wastewater treatment plant, determined the bulk density of the material prior to filtration, which amounted to 950 kg per m3, while the filtered mass contained 25% dry matter and 75% captured water. In preliminary studies, we defined the bulk density in a loose state (after drying at 105°C) which amounted to 700 kg per m3. In this step the composition of untreated sludge was examined (Table 1 and 2).

 

tab01

 

tab02

 

As it can be seen in Table 1, the concentrations of the tested heavy metals are within the allowed limits. This was expected because industrial wastewater has to be primarily treated chemically to ensure compliance with required quality standards for discharge into sewerage system. Also, the results obtained fit into the maximum permitted values of the Directive 86/278/EEC which refers to regulations when sludge is used in agriculture.

A wide number of organic pollutants can be found in the sludge (Harrison et al., 2006; Minini et al. 2015). The effects of their presence are reflected on the soil, vegetation, animal and human health, and certainly affect the environment. The Sewage Directive as well as the Urban Waste Water Directive do not involve monitoring of organic contaminants as in influents and sewage sludge. At this stage of the research, we could not include research for organic pollutants. Setting the limit concentration for organic chemicals is very difficult because of insufficient knowledge on the transformation of organic compounds and the lack of analytical methods. We relied on previously research performed in 2011 (Sarafilovska-Veljanoska, 2011), which includes the Ohrid catchment area where the wastewater treatment facility which is in the function of protecting Lake Ohrid is located.

The EU legislation does not address microbiological problems related to the use of waste sludge. Maximum allowable limits for pathogenic contaminants that pollute waste sludge are not defined. The consequences can be catastrophic in the event that untreated sludge is used as fertilizer. Table 2 shows the microbiological composition of the raw sludge and gives an overview of the effects on selected vegetable cultures treated with untreated waste sludge.

 

Treatment of the Sewage Sludge

The implemented operations for obtaining the ultimate sludge product, which will be used in the production of organic mineral fertilizers, are as follows:

  1. Stabilization and prolonged oxidation - The first phase is carried out in a receptacle in which a perforated pipe system connected to a compressor is installed, which supplies the receiving vessel with oxygen from the air. Also, the system serves to mix the mass. From this vessel, the mass is transported in a reactor where CaO treatment is carried out. The mass becomes greener, rheological characteristics are improved, and a small amount of water is released at the phase boundaries, which is also needed to start the reaction with the lime.
  2. Treatment with CaO - The next operation is the sanitation of the treated waste sludge. The material is transported by screw conveyer to the reactor where the process of sanitization is carried out with lime (Resmi et al. 2012). When transporting from the receiving vessel, conditioning is done and the lime powder is injected. The mixed mass flows into the reactor where the required temperature for the sanitation is achieved. The time to achieve a temperature of up to 100 oC is 30-45 minutes, while the total time of the delay in the reactor is > 2 hours. The liberated energy from the reaction of lime with water is sufficient for complete sanitation. Results from the sanitation process are successful confirmed by the analysis of the samples for microbiological correctness (Table 2).

During the lime treatment, part of the water that is contained in the material is also bonded. When reaching a temperature of about 100°C, part of the water evaporates. Under such conditions, at this stage of treatment, the process of degradation of organic components is prolonged (saponification of the fat components and swelling of the cellulose where part of the water is also bound). Such treated waste sludge that is pathogen free can be deposited without any danger to the environment or can be used for other purposes.

 

Organo Mineral Fertilizer Characteristics

As a final step, the method for the production of organic mineral fertilizers is described by Vait et al. (2012, 2013) and Patent number –P2012/417).The results from the chemical analyses of granulated fertilizer are given in Table 1. Organic matter content has the most significant effect on physical parameters of organo-mineral fertilizers, Allaire et al. (2004 a, 2004 b), Pare et al. (2009). Formulation of gained product, organo mineral fertilizer is 12-16% of organic matter, expressed as dry solids and nutrient content: 1.2% (m/m) of total nitrogen (N), 1.74% (m/m) of total phosphorus expressed as phosphorus pentoxide (P2O5) and potassium oxide 1.1% (m/m) (K2O).

As sewage sludge from different wastewater treatment plants varies in its chemical composition and physical properties, and given the low content of heavy metals from the raw material (Table 1) the heavy metals content does not exceed permissible limits even for organic production. As a result of the whole process obtained from the waste material we treated, in combination with ash from thermal power plants, heavy metals are immobilized even more.

 

Ecological Aspects

If the opportunities for applying the results obtained by this project are found, the effects would be multiple. With the installation of sanitation technology, collected sludge is no threat to the environment. In this way, the possibility of contamination of the environment around the treatment plant is eliminated.

Compared to dried fields treatment, this procedure (P2012/417) does not require large required area, since the norm is 0,06 m2 per population equivalent. Dried field procedure itself is ineffective from the point of constant danger of microbiological contamination, the spread of unpleasant odour, which is reflected in the environment. Also in our case, the risks related to human health originating from the use of waste sludge, including unauthorised users, which may contaminate the garden cultures they produce, is eliminated, (Table 3 and 4).

 

tab03

 

tab04

 

Effects in Agriculture

There are many studies based on the positive effects of using sewage sludge in agriculture, Seleiman et al. (2010), Sohaili et al. (2012), Mtshali et al. (2014). The application of sewage sludge to soil is always associated with limitations related to the presence of heavy metals, toxic compounds, and microbial pathogens. Organo-mineral fertilizer obtained from the waste material we treated, in combination with ash from thermal power plants, has positive effects on the soil. Based on the results from soil samples taken before and after crop harvesting (oriental tobacco and peppers), there is a positive trend in the obtained data from the tested parameters that reflect the quality characteristics of the soil. The content of organic matter in soil increased in both experimental locations and there was no change in trace element content, Pelivanoska et. al (2015), Vait et al. (2012). Also a positive impact was achieved on the agronomic characteristics of tobacco, increasing yield, average price and gross income per unit area, Pelivanoska et al. (2015). In comparison with other literature sources, the content of the studied elements of the collected oriental tobacco, hot peppers (Fortes) and cornichon cucumbers (Table 4) are within the range for plant material, Golia et al. (2009), Gondola and Kadar, (1994), Metsi et al. (2002), Kabata-Pendias, (2011).

Given the above results, we calculated the availability of the nutrients from the filed experiments as a ratio of their available concentration and the total concentration of each element in soil, and given in percentage are 0.1 % for N, 4.9 % for P and 47 % for K.

 

Economic Aspects

With the application of the subject technology, practically two waste materials, waste sludge and ash from thermal power plants receive a certain market value that is effective in the gained product (organo-mineral fertilizer).

An economic aspect of the study was carried out on the basis of the design parameters of a wastewater treatment plant for the protection of Lake Ohrid (Vraništa) with approximately 3 000 t/year of waste sludge production (Vait, 2013). We calculated the cost per EUR/t of dry matter. Only for comparison if the drying fields method is used, the price is 210 EUR/t of dry weight, according to Vait (2013), and the combustion costs are 320 EUR/t of dry mass (Statistics review July 2012, Skopje), while with our technology the achieved costs is 150 EUR/t for the final product (TES, 2013).

 

Conclusions

This paper points out that the production of sewage derived fertilizers can be justified. By stabilization and prolonged oxidation of waste sludge from the Vrasništa municipal wastewater plant we obtained a organo mineral fertilizer product that has many advantages compared with the current technology of based on drying fields. The first benefit is the environmental effect. After treatment with CaO, the use of drying fields is eliminated (except when necessary in emergency cases - equipment damage, etc.). With this type of treated waste sludge, contamination of the environment around the wastewater plant may be eliminated. With this described technology, waste sludge from the wastewater treatment plant and ash from electro filters in thermal power plants receive market value as an organic mineral fertilizer. The effects of the use of organic mineral fertilizers thus obtained are reflected in the increase in the quality of the products and the quality of the products themselves. By comparing the different waste sludge treatment options, we have found that the proposed treatment to be the most economical (150 EUR/t) in terms of unit costs, and far more profitable than other methods.

 

Acknowledgments

This work was only possible thanks to the financial support of the inter-municipal company "Proaqua" Struga. We are grateful to the "Hydrobiological Institute", Ohrid (Department of Physico-Chemical Research and Microbiological Research, 2013) for its unselfish assistance in the preparation of chemical and microbiological analyses and their interpretation. We are grateful to the Scientific Tobacco Institute, Prilep for the analysis of the produced fertilizer and for laboratory and field tests on the basis of which we have built the final product and its benefits.

 

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