Implementation of innovations in waste-to-energy and waste-to-fertiliser processing of the sewage sludge and biodegradable waste, aimed at the reduction of negative impact on the environment in the coastal tourist region

Description

Spółka Wodna “Łeba" (“Łeba” Water Company) was established in 1981 in the coastal town of Łeba at the initiative of municipal authorities. Its main activities are sewage disposal and treatment. The expansion of our town and a neighbouring municipality necessitates further development of the area''s sanitary sewage system and modernisation of the sewage treatment plant. Therefore the aim of our project is to implement innovation in the processing of sewage sludge, and to reduce the negative environmental impact resulting from the touristy character of Łeba and the resulting high seasonality of sewage inflow.

The key benefits to our company and its environment will be: the recovery of electricity and heat by cogeneration through the production of biogas from sewage sludge by methane fermentation (increased energy efficiency of our company), a reduction in the amount and improved properties of sewage sludge sent for composting, and a reduction of odour nuisance (better waste management). Specifically, the innovations will  bring about a reduction of CO2 emissions by 161.25%, a reduction of odour nuisance for 70% of the annual mass of excessive sludge, and a reduction of the emissions of other greenhouse gases (CO, CH4), pollutants and odours, such as H2S and NH3, to the atmosphere.

We will establish cooperation with the Norwegian partner, Aquateam COWI AS, to benefit from knowledge transfer, and the partner’s experience in waste co-digestion. The partner will gain knowledge in the area of seasonal operation of small-scale sewage treatment/microbiogas plants.

Summary of project results

The goal of the project was to implement innovations in the energy and fertilizer recovery from sewage sludge to reduce environmental impact in a coastal tourist region, by introducing a series of process changes at the existing wastewater treatment plant. The main component of the project was to start processing sewage sludge mixed with biodegradable municipal waste through methane fermentation, producing biogas and biofertilizers.

The expected outcomes of the project included:

1. Recovery of Electrical and Thermal Energy based on cogeneration through biogas production from excess sewage sludge via methane fermentation, and additional recovery of electrical energy through the installation of photovoltaic panels for the plant''s own needs.
2. Reduction and Improvement of Sewage Sludge Properties directed towards composting and the development of new alternative fertilizers, thanks to the methane fermentation of excess sludge and subsequent process changes.
3. Reduction of Odor Issues by improving the stabilization of sewage sludge through process changes and methane fermentation of excess sludge.
4. Reduction of Environmental Impact resulting from the high seasonality of sewage influx due to the tourist nature of the location, by implementing technological solutions specifically designed for such conditions.
5. Implementation of a New Waste Stream and the investigation of the practical use of three additional waste streams (as co-substrates) in the biogas production process, particularly those troublesome for tourist towns along the coast (in cooperation with a Norwegian partner and a Polish scientific institution).

Commencement of New Services related to the processing of additional biodegradable waste streams.

1. Innovative Biogas Production Technology in Europe (below 50 kW): The technology involves methane fermentation of mixed excess sludge (previously only composted) and unused co-substrates, which, when combined with the current composting system of the final digestate and biodegradable waste, significantly enhances organic waste recycling in the sewage treatment process.
2. Innovative Heat Recovery Process (National Scale): This process involves the recovery of heat from methane fermentation through a low-temperature district heating system, aimed at increasing energy autonomy for the wastewater treatment plant.
3. Innovative Co-Fermentation Process in Micro Biogas Plants (below 50 kW) (Europe): This innovative process utilizes sewage sludge and other, previously unused substrates for co-fermentation in a micro-biogas plant to treat new waste types, especially in a dynamic coastal tourist region. The goal is to maximize biogas production and reduce the negative effects of seasonality caused by uneven substrate supply during the off-season.
4. Innovative Substrate Quality Control System (National Scale): A national innovation aimed at improving the control of the quality of substrates for biogas production, specifically for wastewater treatment plants with up to 100,000 PE (Population Equivalent). This is achieved through the implementation of a dedicated Production Quality Control Department.
5. Innovative Solution for Coastal and Seasonal Tourist Towns (National Scale): This solution focuses on the education and knowledge transfer related to the operation of micro-biogas plants designed for seasonal wastewater influx. It is intended to demonstrate how such systems can solve seasonal wastewater production problems in coastal and tourist areas, improving water quality in inland and marine environments.
6. Innovative Communication and Education Solution for Waste Separation (National Scale): This solution targets individual and institutional waste producers, educating them about the segregation and preparation of biodegradable waste, such as food waste, kitchen waste, fruit and vegetable scraps, coffee grounds, tea leaves, eggshells, and leftover food.
7. Innovative Mineral-Organic Fertilizer Product (Europe): This fertilizer is based on composted and fermented sludge, with nitrogen recovered in the form of struvite using a new source of ions derived from waste. The goal is to reduce the nutrient load discharged during the tourist season (May to September).

Innovative Green Waste Processing Service (Regional Scale): An innovative service for processing biodegradable green waste, which is preceded by resident education. This service is particularly relevant for regions facing challenges with seasonal organic waste management.

FOR THE APPLICANT (Innovations No. 1, 2, 3, 4, 6, 7, 8)

1. Utilization of available energy potential contained in concentrated excess sludge through sequential methane fermentation (1, 2).
2. Tangible economic benefits through reduced operating costs of wastewater treatment plants in the area of electricity and heat purchase due to energy production from biogas (1, 2).
3. Reduction of dewatered sludge volume through higher dry matter content (minimum 21% dry matter) in fermented sludge directed to composting, owing to the high degree of mineralization of the fermented sludge (1, 7).
4. Economic benefits from the market introduction of new biofertilizers (7).
5. Full control over potential new co-substrates and minimization of risks on a large scale (4).
6. High awareness among residents regarding the cleanliness of new gastronomic-kitchen waste planned for collection (6).
7. High purity of biodegradable green waste planned for collection (8).

Particularly, innovations 1, 3, and 7 will positively impact the volume of processed waste, as maximally specified below, in accordance with current decisions and permits:

Implementation of new co-substrate in the form of biodegradable municipal waste:

WASTE PLANNED FOR METHANE FERMENTATION

• Stabilized municipal sewage sludge received from external sources (Code 19 08 05): Max 3,500 t/year (currently 166 t/year).
• Co-substrate: Other waste (including mixed substances) from mechanical waste treatment other than those mentioned in 19 12 11 (Code 19 12 12) - only the fine fraction of biodegradable municipal waste from mechanical waste processing installations: Max 5,000 t/year.
• Co-substrate: Separated soft (non-woody) fraction of biodegradable waste (Code 20 02 01): Max 200 t/year (currently at this level), and market waste (Code 20 03 02): Max 200 t/year (currently 6 t/year).
• Optionally: Co-substrate: Biodegradable kitchen waste (Code 20 01 08): Max 100 t/year (currently 85 t/year).

WASTE USED IN COMPOSTING

• Post-fermentation residue from municipal sewage sludge, mechanically dewatered from 4% to 21% dry matter on a decanter centrifuge (Code 19 08 05): 2,518.25 / (21%/4%) = 479.7 t/year (own) + imported.
• Separated hard and woody fraction of biodegradable waste (Code 20 02 01): 1,000 t/year, and market waste (Code 20 03 02): 200 t/year.

FOR THE APPLICANT AND THE NORWEGIAN PARTNER (Innovations No. 1, 3, 7)

1. Reduction of negative impacts from the emission of biogenic compounds into the environment caused by the seasonal nature of wastewater inflow, i.e., the generation of significant amounts of sludge during the season, through the recovery of nitrogen and phosphorus in the form of struvite, which releases biogenic elements into the environment at a slower rate. This eliminates emissions from wastewater treatment plants (7).
2. Mitigation of negative impacts from the emission of biogenic compounds into the environment caused by the seasonal nature of wastewater inflow, i.e., the generation of smaller amounts of sludge outside the season, by supplementing sludge with additional co-substrates obtained externally. This eliminates emissions at the source of co-substrate generation (7).

FOR RESIDENTS, TOURISTS, AND THE TOURISM INDUSTRY (Innovations No. 6, 8):

1. Significant improvement in the quality of the sewage sludge stabilization process (through methane fermentation) in terms of its further utilization (increased dry matter content to 21%, transformation of nitrogen into a more assimilable form, and homogeneity) and the reduction of odor nuisance (odorogenicity).
2. The ability to accept other biodegradable waste and increased awareness of higher levels of selective waste collection.

FOR WASTEWATER TREATMENT PLANT MANAGERS IN OTHER TOURIST MUNICIPALITIES IN POLAND AND NORWAY (Innovation No. 5):

1. Knowledge transfer and sharing of best practices in solutions that reduce environmental stress caused by the high seasonality of wastewater treatment plants in tourist regions in the country (5).

FOR THE NORWEGIAN PARTNER

Significance of Innovation No. 3: Co-fermentation Process with New Waste in Norway

There are three popular technologies used to increase biogas production in methane fermentation:

    Thermal hydrolysis process (CAMBI).

    Thermophilic operation mode (55°C).

    Co-fermentation with other organic substrates, commonly used in Norway.

In Norway, the availability of waste resources from fisheries, aquaculture, and fish processing sectors is considered, promoting biogas production and nutrient recovery using such materials (Hamilton et al., 2016; Solli et al., 2014; Gebauer, 2004).

Due to stricter regulations on fish farming in Norwegian fjords, new land-based installations are being planned. The trend favors closed system installations (RAS - Recirculation Aquaculture Systems), which generate larger amounts of sludge requiring appropriate and sustainable management. An interesting alternative is the utilization of sludge and waste in existing biogas plants capable of co-fermentation with other organic waste fractions, such as sewage sludge (Vangdal, 2014).

Significance of Innovation No. 7: Biofertilizer with Slow Nutrient Release

Phosphorus sources (phosphorites) are considered a critical raw material (EC, 2017), even though this nutrient can be found in various waste materials, such as fish sludge and sewage sludge. During the co-fermentation of such substrates, the amount of phosphorus in the digestate increases. However, recent studies show that its bioavailability as a plant nutrient still heavily depends on the quality of the sewage sludge used (Rusten et al., 2018; Estevez et al., 2019b).

Since the 1970s, Norway has practiced using stabilized municipal sewage sludge as fertilizer on arable land. However, recent studies reveal that, contrary to previous assumptions, phosphorus in sewage sludge is not fully available for crops. This is mainly due to the chemical treatment of wastewater in most Norwegian treatment plants, involving aluminum or iron salts to precipitate phosphorus, which remains strongly bound in the final product (Øgaard & Brod, 2016; Krogstad et al., 2005).

Therefore, it is crucial not only to determine how much phosphorus in sludge is readily available to crops but also to prepare fertilizers in forms that allow for slow nutrient release. This involves converting non-absorbable nutrient forms into more accessible ones for plants, with the ability to control availability over time. Proper proportions of recovered struvite and mineral admixtures (ash and dust) are expected to make this possible.

Optimizing the use of available waste resources and recovering nutrients, energy, and biomaterials will be critical for future circular economies. In Norway, approximately 27,000 tons of nitrogen (N) and 9,000 tons of phosphorus (P) are lost annually to the sea as fish sludge—mainly consisting of fish feces and feed surplus (Hamilton et al., 2016).

The scale of biogen losses in fish sludge is comparable to that of livestock manure. Thus, the intensification of aquaculture and its production growth can only be sustainable if fish sludge is recognized as a valuable nutrient resource and collected for subsequent use, e.g., as a mineral fertilizer substitute (Egle et al., 2016).

Using fish processing waste in fermentation not only addresses waste disposal but also enhances the fertilizing value of digestate due to its high NPK content. While fish waste contains proteins that can pose challenges, these can be balanced by adding carbohydrate-rich waste. The nutrient and organic matter content of sewage sludge makes it a highly valuable waste for fertilization purposes. However, the presence of mineral and biological contaminants in sludge may limit its agricultural use.

Fermentation enables hygienization (pathogen reduction) of sludge due to the operating temperatures of biogas plants. This process produces stable digestate rich in nutrients.

FOR THE MARKET (Innovations No. 1, 3, 7, 8)

Although part of the fish processing industry in Pomerania has been phased out, the region still hosts several plants, including 10 in the Tri-City area, 4 in the Hel Peninsula and Lake Żarnowieckie region, 1 in Łeba, and 1 in Słupsk. Simultaneously, there is a growing trend of building aparthotels, with the number of hotel accommodations expected to rise to 12,000 in the coming years. This growth is anticipated to lead to an increase in wastewater discharge.

An additional advantage of the expanding hotel and gastronomy sector will be the need for a greater number of wastewater pre-treatment devices, such as grease separators, in restaurants and other large-scale catering establishments (e.g., corporate canteens, hotels, and large food preparation facilities), in compliance with PN-EN 1825-2 standards.

The applicant has established relationships with a supplier and manufacturer of such devices in the region, ensuring access to additional sources of high-calorie organic fats separated by these grease traps. These fats could serve as an additional co-substrate for powering the micro-biogas plant, increasing biogas production profitability while addressing the problem of excess fat accumulation in separators.

Summary of bilateral results

1.Acquisition of know-how in co-fermentation processes for three additional waste streams: fats, fish waste and gastronomic/kitchen waste from the hospitality and tourism sector in a dynamically developing coastal tourism region. These waste streams, previously untreated, can now be incorporated into future processes to maximize biogas production using new co-substrates. This includes addressing the negative impacts of seasonal variations caused by uneven substrate (sludge) inflows to the biogas plant during the off-season (October–April) by leveraging highly efficient biogas-producing fats from the gastronomy sector.2.Implementation of solutions for a wastewater treatment plant serving up to 100,000 PE (Population Equivalent), including the establishment of a Production Quality Control Department. This ensures monitoring and control of substrate quality, particularly biogas potential, before their introduction to the biogas plant.Task 1: Assessment of Techno-Economic Potential, including the availability of additional, challenging waste streams for co-fermentation in the region. Task 2: Optimization of New Waste Substrate Mixes to Maximize Biogas Production, Considering Seasonal Variability of Sludge Availability. The conducted research demonstrated the potential to enhance biogas yield by adding kitchen BIO waste collected through selective waste sorting or gastronomic waste. Task 3: Estimation of Phosphorus Content in Digestate for Agricultural Use. The analyses indicated that sewage sludge has favorable fertilizer properties. This is evidenced by a high total phosphorus content and a significant proportion of its bioavailable fraction. The bioavailable phosphorus content accounted for approximately 25% of the total phosphorus.The collaboration with the Norwegian partner has been deemed successful, and the company considers the possibility of continuing this partnership in the future. However, there are currently no concrete plans for further cooperation.