Increasing paper mill sludge biodegradation from paper recycling process with biomass liquefaction

Papirna industrija je ena od večjih porabnikov vode in energije na svetu in eden največjih proizvajalcev odpadne vode. Posebno ravnanje z odpadno vodo je zelo pomembno zaradi zaostrovanja okoljskih omejitev za ta sektor industrije. Aerobni, anaerobni in kombinirani sistemi se uporabljajo za čiščenje odpadne papirniške vode. Sodobna uporaba in obdelava papirniškega mulja običajno zajema sežiganje ali uporabo v gradbene namene ter produkcijo encimov. Ker je papirniški mulj biološko skoraj nerazgradljiv smo poskušali s hidrolizo izboljšati anaerobno razgradnjo mulja, kar bi ob produkciji biometana omogočilo energetski povratek procesu utekočinjanja mulja. S tem bi tudi bistveno zmanjšli količino primarnega in sekundarnega odpadnega papirniškega mulja iz odpadne papirniške vode. Namen naloge je bil preučiti postopek utekočinjanja papirniškega mulja v majhnem visokotlačnem reaktorju pri različnih temperaturah, različnih zadrževalnih časih in koncentracijah klorovodikove kisline (HCl). Optimalno utekočinjanje smo dosegli pri 280 ºC z učinkovitostjo 94 % (7 % HCl), sledijo učinkovitosti 91 % (6 % HCl), 87 % (5 % HCl) ter 71 % (4 % HCl). Po predhodnih raziskavah smo predvideli, da bosta anaerobno razgradnjo zavirala toluen in vinil krotonat, ki sta prisotna kot kemikaliji v proizvodnem procesu papirja. Za kvalitativno in kvantitativno določanje prisotnosti toluena in vinil krotonata - kot potencialna inhibitorja biorazgradnje - smo uporabili plinsko kromatografijo sklopljeno z masno spektrometrijo (GC-MS). Rezultati so pokazali da v mulju ni prisotnosti vinil krotonata, koncentracija toluena pa je nihala med 6,5±0,6 in 5±0,8 ppm, s čem smo dokazali da teh dveh inhibitorjev ni v mulju. Nadaljnja obdelava utekočinjenega mulja je potekala z anaerobno razgradnjo, kjer smo uporabili suspendirano in granulirano biomaso. Najboljši rezultat biometanskega potenciala je znašal 182 L/kgKPK, z 52 % biorazgradljivostjo. Z upoštevanjem porabe energije za izvedbo procesa utekočinjenja, je neto kurilna vrednost utekočinjenega mulja v najboljšem primeru znašala 2075 kJ/kg, kar je nižje kot običajna metoda sežiganja dehidriranega papirniškega mulja, ki daje energijsko vrednost med 2800 in 3600 kJ/kg. Pulp and paper industry is one of the most water and energy intensive sectors in the world, as well as one of the largest producers of wastewater. In particular, the wastewater treatment process is very important due to increasing environmental constraints for this industrial sector. Several techniques such as aerobic, anaerobic and combined aerobic-anaerobic treatments have been used to treat the pulp and paper mill wastewater. To-date there are not many options available for utilizing the pulp and paper mill sludge produced, while its handling costs are high. Generally, the sludge is incinerated or finds some limited usage in construction materials and/or enzyme production. Since the paper mill sludge is almost non-biodegradable, we tried to hydrolyze it and make it in its liquefied form more accessible for anaerobic digestion. This would provide biomethane recovery and most importantly, it would significantly reduce the amount of waste produced. The aim of this work was to study the papermill liquefaction process, in a small high-pressure reactor, in relation to temperature, residence time and concentration of hydrochloric acid (HCl). The optimal liquefaction was achieved at 280 ºC with efficiency of 94 % (7 % HCl), followed by 91 % (6 % HCl), 87 % (5 % HCl) and 71 % (4 % HCl). From previous research, we have anticipated that anaerobic process would be inhibited by toluene and vinyl crotonate, which are present in the pulp and paper production as chemicals. For the quantitative and qualitative analysis of toluene and vinyl crotonate in the paper mill sludge - as potential inhibitors of biodegradation - we used gas chromatography coupled with mass spectrometry (GC-MS). Results showed concentrations of toluene between 6.5±0.6 and 5±0.8 ppm and no presence of vinyl crotonate. Further processing of the liquefied paper mill sludge was conducted with anaerobic degradation where we used suspended and granular biomass as inoculum. Best biomethane potential result obtained was 182 L/kg COD with 52 % biodegradation. In respect to the energy consumption for the paper mill sludge liquefaction process, the net calorific value of sludge in the best case was 2075 kJ/kg, which is lower in comparison to the dehydrated paper mill sludge incineration (between 2800 and 3600 kJ/kg), considered to-date as the best available technology.