## 3 Results and Discussion
### Energy Balance: Co-Digestion Case
A microalgae volumetric flow of 410 m\({}^{3}\)/d (TS content of 7%, VS percentage of 91%) was considered to keep previous assumptions consistent. Biogas produced would be, in this case, equivalent to 6253 kW. The thermal hydrolysis unit would demand 1231 kW, leading to the same amount of biogas available as fuel in engines (5022 kW). Electricity production would be increased until reaching energy self-sufficiency (1885 kW), obtaining savings valued at 1,519,160 \(\upepsilon\)/year. However, the application of the microalgae co-digestion strategy requires the installation of an additional hydrolysis unit to cover the higher flow. For both cases studied, the hydrolyzed substrate contains enough energy to provide the thermal demand of digestion. Therefore, the thermal energy available from the engine cooling circuit is not needed. The main cooling circuit has a thermal power of 873 kW in the cheese whey co-digestion case and 778 kW in the micro-algae case with a temperature of the hot fluid stream of 84.2 \({}^{\circ}\)C. This stream could supply domestic hot water and heating to the office building of the WWTP, or be used in a trigeneration system by installing an absorption chiller to cover cooling needs in summer. The auxiliary cooling circuit has similar power for both co-digestion cases, with a hot fluid maximum temperature of only 49 \({}^{\circ}\)C (the approach temperature between fluids was 10 \({}^{\circ}\)C). This value is too low to produce any work. The low temperature of this stream and, therefore, the low amount of energy stored prevent its use as residual thermal energy in any process having direct application in the WWTP. However, given the large quantity of these low sources in urban areas, future research efforts should be performed to find efficient ways for this type of heat recovery [65]. The residual energy of exhaust gases is 981 kW, considering an outlet temperature of 142 \({}^{\circ}\)C, from which just 323 kW are used in the recovery boiler, with the remaining being wasted by emission directly into the atmosphere. If this energy is used to raise the water temperature in the main cooling circuit, the usable power for heating, domestic hot water, or trigeneration, could be considerably increased to 1531 kW. Table 10 presents the results from the energy balance. A similar analysis for the hydrolysis case leads to a slightly lower amount of residual energy contained in exhaust gases. In this latter case, the value is 978 kW, but a greater amount is needed in the recovery boiler because of the increment in steam demand. Thus, 567 kW are used in this unit. The remaining amount (411 kW) is wasted. Just as previously analyzed, if this power were used, the amount available for domestic hot water applications or trigeneration would be 1189 kW. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Sludge + Cheese Whey** &
\begin{tabular}{c} **Sludge + Microalgae** \\ **Biomass** \\ \end{tabular} \\ \hline Biogas production (kW) & 5892 & 6523 \\ Biogas boiler (kW) & 870 & 1231 \\ Electrical power (kW) & 1885 & 1885 \\ Feed water (kW) & 105 & 184 \\ Main circuit (kW) & 873 & 778 \\ Auxiliary circuit (kW) & 406 & 417 \\ Exhaust gases (kW) & 1593 & 1596 \\ Mechanical losses (kW) & 63 & 63 \\ Radiation losses (kW) & 97 & 99 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Energy balance of biogas engines considering co-digestion with cheese whey and with hydrolyzed microalgae. For simplicity, the same biogas yield after thermal pre-treatment was assumed.