Abstract

This paper analyzes the problem of septage management on Croatian islands in accordance with the circular economy framework. The systems approach methodology has been used to analyze management issues. A large number of individual housing facilities on Croatian islands are not connected to the public sewage system. As a solution to the requirements of the competent services, holding tanks are used which are largely permeable. Due to tourism activities the amount of wastewater seasonally varies considerably and in summer is up to 10 times higher than in winter. Such a situation creates major problems for property owners due to mostly uncontrolled disposing of septage, which endangers the environment and human health. Although the situation is improving by application of EU directives, due to poor population density septage management will remain a permanent problem. The EU legal framework requires that organic waste be disposed of in accordance with the principles of a circular economy. This also applies to septage. It has been found that simple and robust options are sustainable and the best choice for a small island environment. After appropriate treatment, septage becomes a resource that strengthens the viability of living on the islands.

BACKGROUND

There are 698 islands in Croatia, of which 48 are permanently inhabited and about 120 are inhabited only in summer during the tourist season. The most densely populated is the island of Krk with 17,860 inhabitants and the least inhabited is Sveti Andrija with one inhabitant.

Islands are isolated and specific natural and socioeconomic systems where sustainable management of wastewater is very important. The most important economic activity on the islands is tourism and that is why environmental security is a priority. However, most individual housing facilities outside large settlements are not connected to the public sewage system. As a solution for their wastewater, owners use an on-site wastewater treatment system, mostly holding tanks (HTs) that are permeable so that wastewater flows underground through the bottom of the tank. Concrete reliable data on the current state of septage disposal on the islands are not known. As an example of the possible state, the known data for the island of Brač can be provided. There are 14,041 inhabitants on the island, and 14,541 housing units and 6,043 HTs (Water Company Brač, 2015). This means that on average about 40% of the houses are connected to HTs. On small islands the number of HTs is significantly higher, up to 100%, because on small islands the sewerage system is mostly not built.

During the summer a large number of tourists stay on the Adriatic islands and in the short period of 3–5 months generate considerably larger amounts of wastewater, so HTs should be emptied frequently. The number of registered overnight stays on the island of Brač in 2017 was 1,697,300 and the actual number was significantly higher, so the number of people during peak days in the season increased by more than 200%. However, when considering the increase of individual housing units in which tourists stay, the increase is up to 10 times bigger (houses with 2–4 apartments). Since HTs are generally not dimensioned for peak summer flow of wastewater, the frequency of discharges of HTs during the season is great, causing big problems and costs for the owners of housing units. At the same time significant quantities of wastewater infiltrate into the ground below the pit and continue to groundwater, which they pollute.

Sludge and wastewater from septic tanks (STs) and HTs, or septage, are partially disposed of directly into local sewage systems and partly illegally on the ground. The septage content discharges into the public sewerage system are generally not regulated in accordance with the rules of the profession, resulting in air pollution, disruptions in wastewater discharge and overloading of the existing sewage system. Because of this, a large number of HTs are emptied illegally, on the terrain at various isolated locations on the islands, which endangers the environmental security.

The current situation is being remedied through the construction of sewerage systems. According to the EU agreement by 2023 most settlements in Croatia with load of more than 2,000 population equivalent will have a regular wastewater drainage system in accordance with the Waste Water Directive (91/271/EEC) (EEC 1991). However, the sewerage network will never include all housing facilities on islands, so the septage disposal system will remain in place. Septage management is a permanent problem and must constantly be upgraded and adapted to the new situation and regulatory framework. This means that it has to be addressed by long-term measures in accordance with the circular economy guidelines.

The paper analyzes the problem of sustainable septage management on islands in accordance with the circular economy framework (EC 2015a, 2015b). It presents an overview of how the sustainability objectives (environmental, economic, and social) of septage management can be met. The problem of septage management has been well documented in the literature and has long been practiced (EPA 1984; WHO 1992; Tilley et al. 2014). However, the issue of septage management on karstic islands in Croatia in accordance with the circular economy framework has not been considered so far.

SEPTAGE GENERATION AND CHARACTERISTICS

There is a difference between septage generation in STs and sewerage HTs, Figure 1.

Figure 1

Septic tank, holding tank and drainfield (modified from Tilley et al. 2014).

Figure 1

Septic tank, holding tank and drainfield (modified from Tilley et al. 2014).

The ST has an outlet pipe to the soil absorption system or some other treatment, while the HT does not have it. An essential difference between ST and HT is in retention time of wastewater within the tank. In sewerage HTs wastewater is retained until the tank is filled. Let us say that for a daily flow of 0.5 m3 (four occupants) and HT volume of 15 m3, the HT has the capacity of storing wastewater for about 30 days, after which it has to be emptied. During the accumulation of wastewater in the tank, anaerobic digestion occurs and greenhouse gases are emitted from the tank (CH4 + CO2). The ST is a passive low-rate anaerobic digester with a specific ecosystem, in which facultative and anaerobic organisms perform complex biochemical processes. The tank operates as a plug-flow type of reactor. The wastewater retention time in the tank is 2–4 days, while sludge is retained for more than 1 year. It is a reactor with high length-to-width ratio in which longitudinal dispersion is small. In the tank there is no mixing or heating. Lighter particles ascend to the surface forming scum, while heavier ones descend and form sludge. As a result, stratification, as well as a clear middle zone, develops. The environment within the clear zone is generally anoxic, while sites within the sludge and scum layers may be completely free of oxygen, or anaerobic. When the maximum permissible amount of sludge has been deposited in the tank, the tank is emptied, under normal load, usually after 2–3 years. As the wastewater passes through the tank, its characteristics change and different bacterial cultures break down complex proteins, carbohydrates and grease. Generally, more than a 65 percent reduction in 5-day biochemical oxygen demand (BOD5), about 75 percent reduction in total suspended solids and about 80 percent reduction in oil and grease occur as the wastewater passes through the tanks with standard retention time of 2–4 days (Bitton 1994).

Tanks are upgraded, all in order to increase their effectiveness (BOD and also microbe removal). Therefore, anaerobic baffled reactors are built. Baffles, under which wastewater is forced to flow, increase contact time with active biomass, which results in improved treatment. Hydraulic retention time is between 40 and 72 hours. In the first part, the easily depositable substances are deposited and those with least grease. As water moves from one part to the other it is lighter and the effluent is cleaner. However, when septage is retained longer in the tank, ST will generate more greenhouse gases (CH4 + CO2).

There is another ST modification option in which the second and subsequent compartments are made as filters and an anaerobic filter is obtained. It is a fixed-bed biological reactor with one or more filtration chambers in series. The recommended hydraulic retention time is 12 to 36 hours. The anaerobic processes in ST reduce the organic and pathogen load, but the effluent is not suitable for direct use. Due to this, effluent is often diverted to the ground through an on-site infiltration system (drainfield: DF).

The HT consists only of one closed chamber (one-piece) and there is no outflow of effluent. So, the main purpose of the tank is collecting wastewater for treatment at another location. They function as batch reactors with variable inflow but with extremely weak mixing. In the tank there is a permanent and increasing accumulation of wastewater and products of organic substance decomposition. In the tank there is partially an anoxic state as a result of constant inflow of fresh wastewater. Due to anaerobic decomposition the tank also continuously generates greenhouse gases (CH4 + CO2).

A permeable HT functions in a specific way as an uncontrolled hybrid of ST and HT systems. In the case of a permeable HT, infiltration of groundwater into the tank and/or exfiltration of wastewater from the tank into underground space can occur, all depending on the groundwater level. In the low coastal area on islands, infiltration of the seawater into the tank has a significant impact on the balance and the biological process in the tank. This is why the situation is different and complex because all variables are unknown. In this case it is practically impossible to make a balance of masses and calculate septage characteristics more precisely. The variations of groundwater inflow and/or sewerage seepage are large and only monitoring can be used to determine average concentration of certain substances as well as balance septage. On karst islands that are built of water-permeable limestone, the aquifer is made of freshwater that floats on sea water or brackish water (mean sea level aquifer). Most of the uncontrollable infiltration of wastewater flowing into the ground beneath the permeable HTs penetrates into this aquifer and pollutes it. Pollution is the most significant in summer when the impact is the greatest and the aquifer has the smallest water volume.

The ground below HT generally has a small capacity to absorb the effluent. Infiltration mainly occurs for example at a soak pit or leach pit infiltration system, where water slowly soaks into the ground, and less as a leach field or drainage field system that consists of a network of perforated pipes that are laid in underground gravel-filled trenches to dissipate the effluent. Because of these processes in permeable HT, the characteristics of septage/sludge to be handled are different and depend on the daily quantity of wastewater which flows into the HT, Table 1. In tourist areas, due to extreme change in the regime of wastewater inflow, the permeable HT has significantly different characteristics in the winter and summer period. In the winter period, due to a longer hydraulic retention time, HT septage has the characteristics of classic ST septage, while in the summer period, due to short hydraulic retention time, it has the characteristics similar to domestic wastewater. Dry solid level in this septage is about 6%. However, septage always has a significantly higher concentration of parameters compared to standard wastewater. Longer septage retention in permeable HT increases the dry solid level, from 6% in liquid–solid to 40% in semi-solid cake, and possibly more in dry solid septage, where dry solid level is greater than 45%.

Table 1

Physical and chemical characteristics of septic tank contents (mg/l) (EPA 1984)

Parameter Average Min Max Variance EPA mean Wastewater 
Total solids 33,800 200 123,860 619 38,800 720 
BOD5 8,343 700 25,000 36 5,000 220 
Chemical oxygen demand 28,975 1,300 114,870 88 42,850 500 
NH3-N – – – – 157 25 
Total phosphorus 155 20 636 32 253 
Grease – – – – 9,090 100 
pH – 5.2 – 6.9 – 
Parameter Average Min Max Variance EPA mean Wastewater 
Total solids 33,800 200 123,860 619 38,800 720 
BOD5 8,343 700 25,000 36 5,000 220 
Chemical oxygen demand 28,975 1,300 114,870 88 42,850 500 
NH3-N – – – – 157 25 
Total phosphorus 155 20 636 32 253 
Grease – – – – 9,090 100 
pH – 5.2 – 6.9 – 

Therefore, septage has to be always adequately treated prior to application of co-treatment options with municipal wastewater, in order to reduce the ‘shock’ effect on wastewater treatment plants (WWTPs). Precisely these septage features lead to the fact that separate treatment is mostly a better solution.

Septage is very rich in nutrients and organic substances and therefore desirable in agriculture and for the production of biogas, Table 1. It is important to emphasize that generally there is no dangerous substance in the septage and that the appropriately treated septage, to eliminate bacterial contamination, is very acceptable for use in agriculture. The sludge recycling in the EU is regulated by Directive 86/278/EEC on the use of sludge in agriculture (EEC 1986). The Directive 86/278/EEC does not include specific requirements for pathogen content in sludge used in agriculture. However, in order to reduce possible health risks related to pathogens, several national regulations have added limitations on pathogen content to standard requirements for sludge quality. A growing problem is micro-contaminants, including pharmaceutical drugs, such as cholesterol-lowering drugs, chemotherapeutical drugs, antibiotics, analgesics, and hormones, that, through the hydrological cycle, end up in tap water, waste water and environmental biocenosis and finally in human organisms. This is a wider problem that will increasingly affect septage management (LTZ 2008).

The biggest problem in use and in land application is bacteriological composition, Table 2. The concentration of bacteria is high and presents a danger for a person who comes into contact with this material or consumes raw products irrigated with this material. Therefore, septage needs to be disinfected prior to use. From the above it can be concluded that septage is a useful resource and after appropriate treatment can be used in accordance with the principles of a circular economy.

Table 2

Indicative values of pathogenic organisms in the contents of septic tanks (EPA 1984)

Parameter Typical range (counts/100 ml) 
Total coliform 107–109 
Fecal coliform 106–108 
Fecal streptococci 106–107 
Pseudomonas aeruginosa 10–103 
Salmonella 1–102 
Parasites: Toxocara, Ascaris lumbricoides, Trichuris trichiura, Trichuris vulpis Present 
Parameter Typical range (counts/100 ml) 
Total coliform 107–109 
Fecal coliform 106–108 
Fecal streptococci 106–107 
Pseudomonas aeruginosa 10–103 
Salmonella 1–102 
Parasites: Toxocara, Ascaris lumbricoides, Trichuris trichiura, Trichuris vulpis Present 

Guidelines for the use of different types of sanitary waters/substances are proposed by the World Health Organization (WHO), Table 3 (WHO 2006).

Table 3

Numerical guidelines for agricultural or aquacultural waste re-use

Waste product Re-use application Guidelines 
Treated wastewater (1) Unrestricted irrigation ≤10–100 EC/100 ml <1 helminth egg/l 
Restricted irrigation ≤105–103 EC/100 ml <1 helminth egg/l 
Localized irrigation ≤106–105 EC/100 ml <1 helminth egg/l 
Graywater (2) Unrestricted irrigation <105–106 EC/100 ml <1 helminth egg/l 
Restricted irrigation <104–103 EC/100 ml <1 helminth egg/l 
Excreta (untreated FS) Agriculture (soil conditioner) (2) <103 EC/g total solids <1 helminth egg/g total solids 
Aquaculture (3) ≤10−6 EC/100 ml ≤1 helminth egg/l 
Waste product Re-use application Guidelines 
Treated wastewater (1) Unrestricted irrigation ≤10–100 EC/100 ml <1 helminth egg/l 
Restricted irrigation ≤105–103 EC/100 ml <1 helminth egg/l 
Localized irrigation ≤106–105 EC/100 ml <1 helminth egg/l 
Graywater (2) Unrestricted irrigation <105–106 EC/100 ml <1 helminth egg/l 
Restricted irrigation <104–103 EC/100 ml <1 helminth egg/l 
Excreta (untreated FS) Agriculture (soil conditioner) (2) <103 EC/g total solids <1 helminth egg/g total solids 
Aquaculture (3) ≤10−6 EC/100 ml ≤1 helminth egg/l 

WHO (2006): (1) Vol. 2, pp. 60, 70; (2) Vol. 4, p. 63; (3) Vol. 3, p. 41.

EC, Escherichia coli; FS, fecal sludge.

SEPTAGE MANAGEMENT AND CIRCULAR ECONOMY

The new EU Circular Economy package clearly recognized that reducing septage and improving septage management are key steps towards increasing the circularity of the island economy. The application of a circular economy will ensure the achievement of objectives related to the efficient use of resources determined under the Europe 2020 strategy for smart, sustainable and inclusive growth (EC 2011a). In its Roadmap to a Resource Efficient Europe (EC 2011b), the Commission proposed a framework for an action plan, highlighting the need for a comprehensive and integrated approach across many policy areas and levels. This refers to all the resources and waste generated by using resources and therefore to septage as well. The aim is to prevent septage as a by-product of wastewater treatment from becoming waste, but it should be a resource that can be used for specific purposes, for example, returning into the nutrient cycle as compost, and/or for producing energy using biogas, etc.

By this, a framework for solving the problem of septage and effluent disposal is set. The defined approach is very promising for the island environment which is isolated from the mainland and mostly poor with nutrient resources. This is especially true for Croatian islands made of limestone, very poor in productive soil and water, especially in the dry summer period, and therefore poorly bio-productive and poor in biocenosis. On the other hand, in summer, due to a large number of tourists, the amount of septage and effluent is large and can be used for various purposes, thus enhancing the sustainable livelihood on the islands.

For septage management there is a whole range of practical alternatives that can be used on islands along with specific modifications adjusted to the sensitive island environment (EPA 1984; WHO 1992; Tilley et al. 2014). These problems are successfully solved by a systems approach and systems analysis (Jewell 1986). The application of a systems approach facilitates determining that a problem exists, refining the problem, generating possible alternative options within a defined set of constraints, and then determining which solution is best according to objective function/stated criteria.

Waste hierarchy is being used in the EU by waste decision makers as the principle to define sustainable decisions. Waste hierarchy is generally applicable to any type of waste. However, it is necessary to ‘refine’ its scope so that it better fits a specific waste stream(s) as is septage waste. The interpretations of waste hierarchy for septage, to address all aspects of sustainability and economic, environmental and social dimensions, have been developed, Figure 2. This hierarchy allows the establishment of relevant septage management options on the islands by a systems approach.

Figure 2

Interpretation of waste hierarchy for septage (author).

Figure 2

Interpretation of waste hierarchy for septage (author).

Prevention of septage is the most preferable option according to such a hierarchy. Prevention of generation of septage and thus the emergence of problems is achieved by connecting to the local sewage system. This is a long-term goal that is not fully achievable under conditions on islands.

Septage can be reduced by various sanitary technologies/devices (waterless systems) or by reduced water use (WHO 1992; Tilley et al. 2014). These are dry toilet, urine-diverting dry toilet, pour flush toilet, waterless pit system without sludge production, waterless system with urine diversion and similar sanitary technologies that are generally not significantly acceptable in the tourism industry in Croatia, although some technologies can be safely applied.

The direct re-use option for septage is not suitable on the islands because of its bacteriological composition and is not environmentally and socially acceptable in tourist areas, so septage has to be treated before use.

If septage generation cannot be prevented or directly re-used, it must be disposed of in a sustainable way, treated and recycled (recovery energy, e.g. via anaerobic digestion) and, as the least favorable option, disposed at landfill or used in biogas production.

Instead of ST and HT, on-site treatment and disposal systems can be used, such as aerobic wastewater treatment systems with DF, biogas system with DF, blackwater anaerobic treatment system with DF, anaerobic baffled reactor, anaerobic filter, waste stabilization pond, and constructed wetland, as well as connection to the communal sewerage systems. The by-product of some of the mentioned technologies is sludge that should be disposed of according to the principles of a circular economy, similarly to septage.

In the selection of the solution it is necessary to apply a methodology that includes evaluation of environmental and economic dimensions of the septage management system, considering social objectives as much as its suitability. The selection of sustainable options includes the use of multi-objective optimization. This methodology provides a decision support framework that identifies which alternative septage management options are optimal. It means that they minimize both objective functions, i.e. environmental impacts and costs, and maximize social impacts. The final decision will always depend on the number of aspects related to decision contexts (e.g. policy and political), Table 4.

Table 4

Typical evaluation criteria of septage systems

Aspects Criteria 
Environmental Impacts on climate change kg CO2-eq; net emission to environment; recycling rate; amount of energy recovered; etc. 
Economic Treatment cost €/t; transport cost €/t; life-cycle cost, etc. 
Social Health and safety of workers; healthy and safe living condition in community; local employment; etc. 
Technological efficiency; safety; reliability; maturity/market readiness; etc. 
Aspects Criteria 
Environmental Impacts on climate change kg CO2-eq; net emission to environment; recycling rate; amount of energy recovered; etc. 
Economic Treatment cost €/t; transport cost €/t; life-cycle cost, etc. 
Social Health and safety of workers; healthy and safe living condition in community; local employment; etc. 
Technological efficiency; safety; reliability; maturity/market readiness; etc. 

The management of septage is a complex task on islands that should be addressed by applying a life-cycle based framework approach to quantitatively assess the environmental and economic sustainability performance (ISO 2006b). In the application of this approach, the choice of the level of life-cycle impact assessment (LCIA) phase is of great importance, particularly the choice of impact assessment models and methods, which influences – among others – the comprehensiveness of the environmental assessment and the indicators associated with each impact category. In a life-cycle perspective, the total cost for managing a particular mass (e.g. 1 t) of septage is given by the sum of collection, transportation and treatment costs, including any existing tax or subsidy (Manfredi & Pant 2013).

Often, due to financial and other constraints, a simplified LCIA approach is mostly used, without using complex impact assessment models. Instead, simple desk-top methods and expert assumptions are used. Without distinction to the limited possibilities to use more detailed analyses and models, it is always important to apply a systems approach to problem solving and to fully consider the problem.

First the problem characteristics are analyzed: determining the quantity of waste and the characteristics of waste/septage, the layout of septic tanks in the area, the characteristics of septic tanks, the features of space and roads, land use plans, potential locations and their features, previous practice, plans and public relations to the problem, legal constraints, etc. Also analyzed are the features of the sewerage system that is the candidate for septage reception, its capacity and features of the WWTP. After that, the development of evaluative criteria is carried out in accordance with the objective function. The next step is the formulation of feasible alternatives. Potential solutions are drafted and their performances are determined, Table 4. Of all possible alternatives, the feasible ones are selected for the concrete case in accordance with the defined set of criteria. Together with the decision makers, decision weights are determined and multi-criteria analysis and ranking of possible options are carried out (Figueira et al. 2005).

ALTERNATIVE SOLUTIONS REVIEW

A septage management system is a collection of four basic components that are connected by different types of interaction or interrelationships:

  • septic system and on-site management;

  • pumping and haulage management: pumping, transport, transfer station;

  • receiving station, treatment;

  • re-use/disposal.

These system components collectively respond to a given input (septage) to produce the desired output (recycle of materials and recovery of energy). The system and its components are subject to different constraints which set a boundary of the system. The main input into the system is wastewater and the output is effluent, scum and sludge. Other inputs are related to system operation such as energy (fuel, electricity), chemicals, human work and financial resources. Other outputs are gases, noise, vibration, climate impacts, local employment, health conditions, and the like.

The key processes in the system relate to: storage of wastewater, transport, treatment of septage and the use of the final product (e.g. compost) or disposal. These processes cause changes in water content (septage) and characteristics. The process takes place at several locations between which water and sludge are transported in various ways, by pipes/sewage, by gravity force, tanker trucks, and pumping which requires certain power tools and energy while simultaneously causing certain emissions into the environment. Water and sludge are retained in the system for balancing the input and output processes and for treatment. Everything takes place over a certain period of time during which the septage decomposition process takes place continuously. Processes in the system constantly affect the natural environment (water, air, soil, and climate) and socio-economic conditions (health, economy, standard of living, safety, employment, etc.).

The most problematic and demanding component of the system is the treatment of septage that has a significant impact on the choice of solutions. This is especially true for small islands where transport of septage takes place at very short distances and therefore haulage management has no significant impact on the choice of solutions. On larger islands, where distances are large, haulage management is a significant component of the system cost and impacts and it significantly affects the choice of the location of the treatment plant. The treatment of septage can be co-treatment or independent treatment. Direct land application of raw septage on islands is not allowed in Croatia. Land application can only be realized within the process of re-use of treated septage at an independent treatment plant or as treated sludge from a communal WWTP (EEC 1986). Septage, as well as sludge from WWTP, should be recycled or used for energy production as is done with bio-waste. The guidelines of the circular economy predict that after treatment septage be returned to the nutrient cycle and/or used as an energy source.

Treated sludge foreseen for land application must comply with the regulations (EEC 1986). Land application must not endanger groundwater, the natural environment or human beings. Therefore, treated sludge must be of good quality, and soil where it is applied should have good absorption characteristics (slopes, impervious layers, and vegetation). The location for use/disposal must be at an appropriate distance from the source of potable water, surface water, buried drinking water holding tank, community wells, and the seasonal high groundwater level, as well as from a road, driveway, or any buildings. The location should not be used by any future vehicular traffic (driveways, parking or storage areas, bike trails). Land application can be used for agricultural land, forested land and/or reclaimed land. Each application requires meeting special conditions. Land application is a socially sensitive activity which the local population generally does not support.

Effluent from treatment plants is released into water resources or re-used. Effluent from the secondary level treatment plant is disinfected and can then be used for irrigation if it meets the statutory characteristics. Purified water contains dissolved nutrients that are useful for plants. In this way, by the use of the effluent, a part of the nutrients is returned to the food chain.

The energy use of sludge is simple to apply by producing biogas in an anaerobic digester. Biogas production and conversion into energy emits harmful gases that should be controlled. In anaerobic digestion complex organic substance is transformed into biogas (CH4 + CO2) and biomass. Up to 75% of the organic fraction is transformed into biogas containing 50–60% of methane. Biogas usually has calorific value of about 22 MJ/m3 and the thermal value of methane is 36 MJ/m3. As the standard value of transformation efficiency of biogas into electricity is about 35%, 1 m3 of biogas gives 2.14 kWh of electricity. It is green energy whose use is encouraged. Anaerobic digestion is more profitable in larger quantities and is therefore a good solution for large systems. After disinfection stabilized biomass can be used for various purposes.

The greatest value for the environment and sustainability as a whole is the recycling of nutrients into the food chain, mainly for composting (most commonly with other household organic solid waste) and land application/disposal of sludge from septage treatment or co-treatment plants, e.g. aerobic or anaerobic digestion. Organic waste can be applied to crops as fertilizer or soil conditioner, but crops normally take up the inorganic forms of nutrients such as nitrates and phosphates. Therefore, it is more efficient for agricultural re-use to use stabilized organic waste (LTZ 2008). The WHO guidelines for the use of waste products are listed in Table 5 (Eawag 2008). The similarity of characteristics of septage and municipal wastewater makes co-treatment suitable for septage treatment and disposal.

Table 5

Recommendations for agricultural re-use waste products (Eawag 2008)

Waste product Recommendations for agricultural re-use 
Treated wastewater Only water subject to secondary treatment (i.e. physical and biological treatment) should be used. The use of drip irrigation can significantly reduce contamination of root crops and leafy vegetables growing just above the ground, especially crops not in contact with the soil (e.g. tomatoes). The use of spray irrigation systems can also reduce crop contamination. However, a buffer zone of 50–100 m to residents should be maintained. An increase in the period between irrigation and consumption will reduce the danger of crop contamination (0.5–2 log units/d). Washing, disinfecting, peeling, and cooking of fruit, crops or vegetables effectively reduce health risk to consumers. 
Graywater Direct re-use of untreated graywater in irrigation is not recommended. Irrigated graywater should undergo at least primary treatment (e.g. septic tank). Irrigated soil can act as a natural secondary treatment step. 
Excreta Excreta and fecal sludge should be treated prior to their use as fertilizer, and the treatment methods should be validated. Equipment used, for example when transporting unsanitized feces, should not be used for the treated (sanitized) product. Precautions with the handling of potentially infectious material should be taken when applying feces to the soil. These precautions include personal protection, hygiene and hand-washing.
Treated excreta and fecal sludge should be worked into the soil as soon as possible and not left on the soil surface. Inappropriately sanitized excreta or fecal sludge should not be used for vegetables, fruit or root crops consumed raw, except for fruit trees. A storage period of at least 1 month should be applied for treated excreta and fecal sludge. 
Waste product Recommendations for agricultural re-use 
Treated wastewater Only water subject to secondary treatment (i.e. physical and biological treatment) should be used. The use of drip irrigation can significantly reduce contamination of root crops and leafy vegetables growing just above the ground, especially crops not in contact with the soil (e.g. tomatoes). The use of spray irrigation systems can also reduce crop contamination. However, a buffer zone of 50–100 m to residents should be maintained. An increase in the period between irrigation and consumption will reduce the danger of crop contamination (0.5–2 log units/d). Washing, disinfecting, peeling, and cooking of fruit, crops or vegetables effectively reduce health risk to consumers. 
Graywater Direct re-use of untreated graywater in irrigation is not recommended. Irrigated graywater should undergo at least primary treatment (e.g. septic tank). Irrigated soil can act as a natural secondary treatment step. 
Excreta Excreta and fecal sludge should be treated prior to their use as fertilizer, and the treatment methods should be validated. Equipment used, for example when transporting unsanitized feces, should not be used for the treated (sanitized) product. Precautions with the handling of potentially infectious material should be taken when applying feces to the soil. These precautions include personal protection, hygiene and hand-washing.
Treated excreta and fecal sludge should be worked into the soil as soon as possible and not left on the soil surface. Inappropriately sanitized excreta or fecal sludge should not be used for vegetables, fruit or root crops consumed raw, except for fruit trees. A storage period of at least 1 month should be applied for treated excreta and fecal sludge. 

However, septage has to be considered as strong wastewater and appropriate facilities are needed at sewage treatment plants to receive, pre-treat, and distribute the septage into the appropriate process units. This is applied in the case of smaller quantities of septage which do not significantly alter treatment processes in the receiving treatment plant. If septage is strong, usually the sludge stream of a communal WWTP is added to it. If it is weak and similar to wastewater, then the liquid stream of a WWTP is added. Both streams can be used depending on the pre-treatment of septage.

Independent treatment and management of treatment products is a common solution if the quantity of septage is large or if there is no sewerage system at an acceptable distance that could accept it. Different processes can be used for septage treatment: lagoons, secondary biological treatment processes, aerobic digestion, anaerobic digestion, composting, and disinfection.

Depending on the required capacity and working conditions, certain processes and combinations associated with the treatment of sludge and by-products of purification (odor control, dewatering, disinfection, etc.) are applied. Different options can be used for septage management, Table 6. It appears that there are a number of options that can be used. Each of them has its advantages and disadvantages to be taken into account when ranking options to obtain a viable solution. By using the systems approach and multi-criteria analysis, such a solution can be selected.

Table 6

Basic septage management options

Management options  
Land disposal Land spreading
Trench/lagoon/landfill buried
Subsurface incorporation 
Co-treatment Addition to liquid stream
Addition to sludge stream
Addition to both streams 
Independent treatment Waste stabilization ponds/lagoons
Aerated pond
Composting; co-composting (organic solid waste)
Conventional biological treatment
Aerobic digestion
Anaerobic digestion
Disinfection (lime, chlorination) 
Management options  
Land disposal Land spreading
Trench/lagoon/landfill buried
Subsurface incorporation 
Co-treatment Addition to liquid stream
Addition to sludge stream
Addition to both streams 
Independent treatment Waste stabilization ponds/lagoons
Aerated pond
Composting; co-composting (organic solid waste)
Conventional biological treatment
Aerobic digestion
Anaerobic digestion
Disinfection (lime, chlorination) 

RECOMMENDATIONS

In accordance with the presented analyses, the strategy for septage management on the islands in Croatia has been developed, Figure 3. Each island has to be analyzed separately in accordance with the proposed methodology and strategy and an appropriate solution has to be determined. In addition, to try to reduce/prevent septage generation, it is also necessary to design and implement measures to improve the management of septage, so as to reduce unwanted consequences at environmental, economic and social levels.

Figure 3

Recommended local concept for septage management at the islands (modified from Tilley et al. 2014).

Figure 3

Recommended local concept for septage management at the islands (modified from Tilley et al. 2014).

In accordance with the proposed septage hierarchy concept (Figure 2) the most favorable action should be to avoid the generation of septage. The generation of septage is best avoided by building a sewerage system. If this is not feasible, then the most important goal is to avoid the construction of HTs because their maintenance is very expensive. The use of HT implies that wastewater from HT be transported to the local WWTP, if any, or to a mainland WWTP. A better solution is the construction of ST with appropriate DF. DF must be well designed to ensure reliable treatment of effluent from ST. Generally there is very little appropriate land on the islands where septage can be safely disposed. So, direct re-use of untreated septage is not acceptable since it endangers/pollutes groundwater and may cause spread of disease.

Septage has to be treated. Generally speaking, each island has three basic alternatives:

  • local management;

  • transport to the mainland;

  • integration and proper treatment with other organic waste, WWTP sludge, food and green waste on the island.

Starting from the sustainable development strategy of the island, transport to the mainland for treatment with mainland organic waste is considered the least acceptable. The island thus loses an important resource and the cost of storing before transport and handling on land is high.

Integration with other organic waste, WWTP sludge, food and other organic waste is considered the best alternative for the islands. Joint management with other organic waste at the island level is considered the most acceptable solution from the point of view of island sustainability or livelihood sustainability. In all other cases, the problem should be solved locally in accordance with septage quantity and composition, and other factors influencing the septage management system.

The basic septage management options which have to be considered are as follows.

  • Recycling of nutrients and energy recovery are acceptable options in the waste-septage hierarchy and therefore need to be applied. These options contribute to reduction of greenhouse gas emissions, enhance the local nutrient and water cycle in the local natural environment, and enhance local water and food resources and therefore sustainability.

  • Organized disposal of unavoidable septage into engineered land disposal is the least favorable option (isolated trench, lagoon, and landfill burial). On islands such an option (e.g. good management and use of lagoons on acceptable isolated locations) is hardly feasible, although not impossible.

  • Building own treatment plants for septage can be a very useful solution if it can be sustainable. This primarily works on composting in the case of smaller quantities and anaerobic digestion in the case of larger quantities of septage. Both options could be a viable solution and socially acceptable. This is an option for old/heavier septage produced after a long detention period in tanks.

  • Co-treatment is an option for septage from HT and septage from ST with a short detention period in the tank. Such solutions are feasible in the case when larger sewage systems and a WWTP exist on the island. Co-treatment is a good option for larger islands with larger settlements with a bigger number of permanent residents. On such islands, the WWTP is usually designed with secondary level of treatment. The mixing ratio of septage with liquid or sludge in the WWTP must be acceptable in order for the solution to be technologically and economically viable. On small islands only primary level of wastewater treatment is necessary in accordance with wastewater directives. Such treatment plants produce raw sludge which has to be treated jointly with septage.

If there is no existing WWTP then use of lagoons as a solution (see Figure 4) for septage treatment is recommended. Lagoons are simple, robust and reliable treatment plants suitable to balance peak loads. Lagoons produce good quality effluent ready to be used for irrigation. Sludge is very stable and can be used directly as wet material or composted and used as dry material or composted together with other organic waste. Effluent can be used directly in summer when demand for irrigation is large. Sludge can be used in winter and spring when demand for fertilizing is higher (Hellagrolip SA 2018).

Figure 4

Proposed solution of septage treatment for different type of septage.

Figure 4

Proposed solution of septage treatment for different type of septage.

The application of anaerobic digestion and energy generation is a problem on small islands due to high seasonal variation of load and generally small quantities of septage, especially in the winter period when there are no tourists. It is a too expensive technology for small quantities. However, it is a promising alternative which should be considered on big islands in the case of larger amount of septage and in the case of joint treatment septage, WWTP sludge and food waste. Anaerobic digestion generates methane gas which can be used for energy generation, while stabilized sludge should be disinfected and then it can be used as soil conditioning or as fertilizer. Effluent after appropriate treatment can be stored (deep lagoons) and re-used or discharged into soil by a drainfield system.

Additionally it is important to implement appropriate organization of work and system management including an appropriate information–decision system. In order to make the system reliable and effective and to achieve the expected results deriving from the circular economy policy, the concept of a closed-sequence or closed-loop control system should be implemented. Such a concept consists of feedback (corrective) information that is needed for the correction of the system process and outputs. The basic resources of the process have been identified as being water, energy, and material, including operational information and equipment necessary to convert septage into useful outputs (compost, energy). This was taken into account in the selection of solutions to make the system sustainable in isolated environments, such as small islands that lack trained staff to manage complex and sophisticated technological systems. Easy-to-manage solutions which are less sensitive to changes are more appropriate for such an environment as the process control system is simpler and more reliable.

CONCLUSION

Septage management is a permanent issue on the islands. Solving this problem is possible in a variety of ways. Simply, septage should be addressed by taking into account the principles of waste hierarchy. Given that islands are specific natural and cultural environments and that they are sufficiently socio-economically different, it is always desirable to make a simplified analysis of the problem of septage management in accordance with the proposed systems approach methodology and recommendations. It simply implies an analysis of the whole system (all processes involved) rather than its parts only. The objective to be achieved is a sustainable solution for management/disposal of septage.

Septage management is an important activity on the islands and should be implemented in accordance with policy and objectives of a circular economy. Of course, the identification of specific objectives related to economic, environmental and social goals is a part of the planning process. The selection of sustainable options includes the use of multi-objective optimization. No matter how big an island is, the problem is solved by applying a systems approach methodology that takes into account and/or includes a broader framework for solving problems. It includes the environment of the system, the competitive system, and the internal system elements. These are arranged in a hierarchy, with the broadest system being the environmental system.

A good choice of septage solution results in septage after appropriate treatment becoming a resource that strengthens the viability of living on the islands. It can be solved independently, but also together with other organic waste on the island, primarily with wet food waste. The total organic waste of the island can be re-used as fertilizer and thus significantly strengthen local agricultural production. In such a way nutrient resources are naturally integrated into the annual cycle of agricultural activities. Simple and robust treatment options are sustainable – and the best choice for a small island environment.

The presented overview and suggestions can be useful for resolving issues of septage management in other areas, not only small karstic islands. However, appropriate adaptation is necessary in accordance with the local natural and socio-economic framework.

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