# Efficiency of Chemical Toilet Sewage (CTS) Co-Fermentation with Typical Energy Crops
## 1 Introduction  
Certain regions of the world have seen enormous tourist traffic due to their attractiveness, natural beauty, and educational value [1, 2]. One such site is the Great Mazurian Lakes mesoregion in north-eastern Poland [3]. The area spans 1732 km\({}^{2}\), of which 486 km\({}^{2}\) is composed of lakes and 1150 km\({}^{2}\) of forests. This extremely attractive complex stands out among others of its kind in Europe, with a rich network of water bodies and forests, drawing six-digit numbers of hiking, biking and water sport enthusiasts each year [4]. Statistics show that tourist traffic in the region is consistently rising, having reached 1.5 million people per year [5]. Approximately 20,000 yachts manned by nearly 100,000 sailors take to the waters of the Great Mazurian Lakes throughout the tourist season [6]. About 80% of all boats are equipped with chemical toilets, which collect waste that must be emptied regularly and disposed of [7]. Many of the regional campsites serving nearly 200,000 visitors have no sewerage, meaning that they also rely on chemical toilets and the temporary storage of fecal matter in esspoolos [8]. The tourist pressure on the region is not distributed evenly over the year. Nearly 90% of the region's tourist traffic is relegated to the two months of July and August, due to the preferable weather and the school-free summer season [9]. This leads to the high generation of waste, has a significant impact on the environment, and presents an environmental hazard [10]. There are certain substances used in chemical toilets to neutralize odor, improve waste solubility, and dewater the feces. These include: active biocides, oxidizers, thickeners, chelating agents, odorants, surface-active substances, buffers and colorants [11]. Products designed for chemical toilets are made with formaldehyde, glutaraldehyde, quaternary ammonium compounds, inorganic salts, andmost importantly, the powerful biocide bronopol, to imbue them with strong bactericidal properties [12; 13]. Bronopol (2-Bromo-2-nitro-1,3-propanoliol) is not highly toxic to mammals, but acutely toxic to bacteria (particularly Gram-negative ones) and microalgae such as _Scendesmus_ sp., _Nostock_ sp., and _Chlorella_ sp. In fact, this toxic effect can be observed at concentrations as low as 6 to 50 ppm [14]. Applying the OECD 201 test to three microalgae species (_Selenastrum capricornutum, Scenedesmus subspicatus, Chlorella vulgaris_) showed that bronopol doses of 0.4-1.9 mg/L caused a decrease in biological activity for 50% of the population [15]. The abiotic degradation of bronopol is affected by multiple factors, including temperature, pH and light intensity [16]. Water-dissolved bronopol becomes more labile under alkaline conditions and at high temperatures [17]. The half-life of 300 ppm in distilled water is four months at 22 \({}^{\circ}\)C and pH 8, but over five years at pH 6 and pH 4 (temperature being the same) [18]. Due to its properties, toxicity and resistance to degradability, as well as the lack of sufficiently sophisticated wastewater treatment systems and developed sewage systems, other ways of handling such waste need to be identified. Anaerobic digestion (AD) is, potentially, one such solution. It can be used in less-urbanized areas as a fully self-contained system, providing electricity and heat for domestic use and for tourist facilities [19]. The approach is fully in line with the principles of the circular economy and bio-economy, while also contributing to the push towards renewable energy [20; 21]. Biodegradable organic waste collected in chemical toilets may potentially be used as an organic feedstock for AD [22]. However, due to its content of other hard-to-biodegrade and toxic chemicals, this type of waste needs to be reliably tested for co-fermentability with other common types of biomass. Identifying feedstock compositions that can be efficiently digested using AD should be a priority in this regard.