# Sewage Sludge Derived Materials for CO2 Adsorption
## 3 Results and Discussion
The adsorption capacity, separation efficiency and CO\({}_{2}\) recovery were calculated with the following equations:  
\[a=\frac{Q*p\left(c_{i}-c_{f}\right)*t}{m} \tag{1}\]  
where: \(Q\) = the flow of the test gas passed over the adsorbent materials, cm\({}^{3}\)/s; \(p\) = adsorption pressure, bar; \(c_{i}\) = CO\({}_{2}\) concentration in the cylinder (feed gas), vol.%; \(c_{f}\) = CO\({}_{2}\) exit concentration up to adsorbent saturation, vol.%; t = breaking time, s; \(m\) = the amount of the adsorbent materials in the reactor, g.  
\[\eta=\frac{c_{a}}{c_{i}}*100 \tag{2}\]  
where: \(c_{a}\) = adsorbed CO\({}_{2}\) concentration (vol.%); \(c_{i}\) = initial CO\({}_{2}\) concentration (vol.%)  
\[\text{R}_{\text{CO}_{2}}=\frac{\overline{C}_{e(\text{CO}_{2})}\ -\ \overline{C}_{i(\text{CO}_{2})}}{C_{i(\text{CO}_{2})}}*100 \tag{3}\]  
where: \(\overline{C}_{e(\text{CO}_{2})}\) = the average of the CO\({}_{2}\) concentrations until the breaking period at the exit from the adsorbent materials (vol.%); \(\overline{C}_{i(\text{CO}_{2})}\) = initial CO\({}_{2}\) concentration (vol.%). The feed gas flows continuously through installation and every 8 min the output gas was analysed in order to determine the CO\({}_{2}\) content. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Gas Mixture**} & \multirow{2}{*}{**Injection Number**} & \multirow{2}{*}{**Time (min)**} & \multicolumn{2}{c}{**CO\({}_{2}\)**} & \multirow{2}{*}{**a (t)**} & \multirow{2}{*}{**n (t)**} & \multirow{2}{*}{**R (t)**} \\  & & & & **(vol.\%)** & & \\ \hline  & 1 & 8 & 51.19 & 11.87 & 99.68 & 98.11 \\  & 2 & 8 & 38.74 & & & \\  & 3 & 8 & 22.23 & & & \\  & 4 & 8 & 16.88 & & & \\  
As it can be observed (Table 5), in the case of the gas used first, rich in CO\({}_{2}\), the saturation occurred after seven injections. The single run time analysis was 8 min, and 12 runs were performed consecutively, until the initial concentration of CO\({}_{2}\) in gas was noticed. Then, desorption was achieved through vacuum and the adsorption tests continued with the gas mixture with low concentration of CO\({}_{2}\). Even up to five runs, the CO\({}_{2}\) was lower than detection limit. It should be noted that in the case of the second gas mixture, the saturation did not occur even after 100 injections, therefore the degree of recovery was not estimated. In comparison with other studies [36; 37], the adsorption capacity of the adsorbent was similar, even for feed gas with significantly lower content of CO\({}_{2}\) (35%) than in this study. It was demonstrated that not only the activation agent influences the adsorption capacity but also the pyrolysis temperature and the heating rate can modify the adsorption [40]. Thus, a higher adsorption capacity was reached by using higher pyrolysis temperature (800 \({}^{\circ}\)C). Additionally, the combination of sewage sludge with wood sawdust revealed similar CO\({}_{2}\) adsorption capacities, with the values varying with the pressure [54]. From an economic point of view, the technology could be efficient due to waste recycling for value-added applications in concrete CO\({}_{2}\) adsorbents that avoid the expensive sewage sludge disposal. We calculated the techno-economic aspects of sewage sludge conversion, with this becoming economically attractive when the value of avoided CO\({}_{2}\) emissions reached 30 EUR/t. These results are consistent with the literature, which mentions values of approximately 29 EUR/t [55]. Nowadays, the price on the market of European Union Allowances is about 25 EUR/t CO\({}_{2}\)[56].