_Applied_  
_sciences_  
_Article_  
**The Possibility of Using Spent Coffee Grounds**  
**to Improve Wastewater Treatment Due to Respiration**  
**Activity of Microorganisms**  
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coffee production increased by 17% [1]; (ii) in total, 0.91 g wasted grounds per gram of coffee is produced. Around 6 million tons of spent coffee grounds are produced each year [2]. Consequently, these facts have made many research teams focus on the usage of spent coffee grounds [3]. On the other hand, the release of spent coffee grounds to the environment due to increasing coffee consumption causes environmental contamination due to the fact that its decomposition consumes large quantities of oxygen [3]. All waste disposal issues represent big economic and environmental problems [4]. SCG utilization and management represent a big problem due to the presence of phenols, caffeine, and tannins, which are highly toxic for many life processes. The study showed that the toxicity of SCG could be reduced by warm treatment, microbial biodegradation, and aerobic fermentation [5; 6; 7; 8; 9]. SCG is most often part of municipal solid waste, so it can be incinerated or landfilled [10]. SCGs, on average, contain 45.3% (w/w, dry weight) of polysaccharides (mannose, galactose, glucose, and arabinose) bound to cellulose and hemicellulose complexes [11; 12]. Mannans are the major component of polysaccharides in SCGs [12]. There are almost no reducing sugars present in coffee [13]. The main phenol presented in coffee is chlorogenic acid (CGA) and accounts for up to 14% (dry matter basis) [14].  
Phenolic compounds, flavonoids, chlorogenic acid, and protocatechuic acid are important for their antioxidant activity [15]. Chlorogenic acid is one of the phenolic compounds found abundantly in SCGs [16; 17]. Phenolic compounds can be toxic, but only for some aerobic microorganisms and only in high concentrations [18]. Microorganisms can isomerize, hydrolyze, or degrade them into low-weight molecular compounds. On the other hand, high concentrations of phenolic compounds have an adverse effect on anaerobic microorganisms, which may cause problems at the processing of biodegradable waste in biogas transformation technologies [19; 20; 21; 22]. Phenolic compounds are a major cause of digestive failures in anaerobic digestion [23]. The literature also describes the use of SCGs as a cheap raw material for removing dyes and heavy metals from wastewater [24]. There should also be attention focused on the previous research conducted on the valorization of spent coffee grounds as a way to reduce dyes and heavy metals from wastewater, which have reported some yield and the weak points of the proposed processes. Wastewater is polluted by a wide range of pollutants, such as carbonaceous substances or heavy metals, which access wastewater treatment plants (WWTP). The sludge is made up of the entire spectrum of microorganisms and protozoans that make up the community effectively degrading these pollutants. The differences between SCGs are related to how they are prepared, from different ratios of Robusta (_Coffea caenphora_) and Arabica (_Coffea arabica_) coffee, as these coffees are commonly labeled [25; 26]. The possibility to use of SCGs in wastewater treatment has not been investigated or published yet. There is no information about the influence of SCGs on the respiration activity of microbial communities in the literature. Data on possible SCG aerobic degradation or utilization by a sewage sludge bacterial consortium have never been reported either. The aim of the research was to determine the influence of spent coffee grounds on microbial respiration in the activated sludge and establish differences in the compound profile of unprocessed, processed, and filtrate samples.