A contribution to resource recovery from wastewater. Anaerobic processes for organic matter and nitrogen treatment

Abstract

Organic matter and nutrients present in urban and industrial wastewater should be removed or valorised to reduce its impact on the environment. Conventional wastewater treatments are focused on the removal of these pollution sources at the minimum cost. The idea of resource recovery from wastewater is changing the concept of the conventional wastewater treatment plants that tend to incorporate little by little processes as anaerobic digestion, MBR, biofilm, granulation, etc. However, their application to obtain reusable by-products from wastes should be cost effective. Winery wastewater is an effluent with a highly biodegradable organic load, worth considering for biogas production, and low nutrient content. A sidestream anaerobic membrane bioreactor (AnMBR) was started up and operated under organic load oscillations. The stable operation was assured by keeping a ratio between intermediate alkalinity and total alkalinity (IA/TA) below 0.3 achieving a 96.7±2.7% COD removal efficiency. The biogas production varied according with the OLR that was on average 0.50±0.17 m(3)(biogas)/m(-3)(digester)/d(-1) with an 87.1±3.0% of methane. The external membrane module reached a flux of 20.2 ± 8.5 LMH at a crossflow velocity of 0.64 m s-1. The crossflow velocity helped to remove the cake layer attached on the membrane that was the main contribution (>80%) to flux decline. The energy demand of the AnMBR was calculated considering the net energy production of a combined heat and power (CHP) unit. It was concluded that only when influent COD was over 3.25 gCOD 1:1- the energy balance was positive. Considering the upscaling of the AnMBR of the present study, the submerged membrane configuration would be a more feasible option due to its lower operational costs, especially in winter season. Simulating the conditions of winter season, winery wastewater was treated in the AnMBR at low temperatures of 25°C and 15°C. Since the organic load of winery wastewater in winter is much lower than in summer (vintage season), the average OLR applied was 0.32 and 0.29 kgCOD m(-3)(digester)/d(-1) the average COD removal reached was 80% and 71% at 25°C and 15°C., respectively. Moreover, higher degree of fouling was observed despite the amount of suspended solids was lower, so frequent cleanings were necessary. The effluent of anaerobic digestion often requires a post-treatment to remove nutrients, especially nitrogen. Compared with conventional biological nitrogen removal, nitritation/denitritation (N/DN) via nitrite represent a 25% less aeration and 40% less external carbon source. In order to reach higher flexibility and reducing space requirements, N/DN can be carried out in a sequencing batch reactor (SBR). Under feast and famine conditions applied in the SBR, storage compounds can be served as internal carbon sources for post-anoxic denitrification. A novel scheme was developed for the treatment of municipal wastewater; consisting in an UASB reactor followed by a short cut sequencing batch reactor (scSBR) in the main water line. Nitritation/denitritation was integrated with the selection of polyhydroxyalkanoate (PHA) storing biomass. Biowaste fermented liquid was applied as carbon source in the feast regime. The average nitrogen removal was 83%. After achieving an acceptable ammonia removal of 93%, there was enough available PHA for the subsequent denitritation, reaching a maximum nitrite removal of 98%. The maximum PHA content was 10.6% (gPHA gTSS-1) after 10 h of accumulation when biowaste fermented liquid (C/N/P= 100/4.5/0.42) was applied. Nitrogen removal limits could be successfully met while PHA-storing biomass was selected. Although higher PHA yields can be achieved under complete aerobic conditions, this novel scheme presents an added value due to the integration of the PHA production in the nitritation/denitritation process.

Un dels reptes de les estacions depuradores d'aigües residuals (EDAR) és aconseguir una elevada eficiència d'eliminació de contaminants amb menys demanda energètica, atenent als requeriments legals, cada dia més restrictius. A més a més, l'afany de recuperació de recursos a partir dels residus promou el desenvolupament de noves tecnologies pel tractament d'aigües residuals valoritzant-ne la matèria orgànica i els nutrients que conté.

La digestió anaeròbia és un procés pel qual la matèria orgànica es transforma en una font d'energia, com és el biogàs. Actualment, la digestió anaeròbia s'aplica a residus com els fangs de depuradora, purins, etc. En aquesta tesi, s'ha estudiat la digestió anaeròbia per al tractament d'aigües residuals amb poca càrrega orgànica, utilitzant bioreactors de membrana anaeròbics (BRM-An) per tal de retenir la biomassa en el digestor. S'ha tractat aigua residual procedent de la indústria vitivinícola, caracteritzada per un alt contingut en matèria orgànica i pobre en nutrients, avaluant la flexibilitat del BRM-An a oscil•lacions de càrrega orgànica, així com la seva viabilitat en termes energètics. Pel que fa a l'aigua residual municipal, la digestió anaeròbia requereix de post-tractaments d'eliminació de nutrients. En aquesta tesi, s'han estudiat dos processos d'eliminació de nitrogen: nitrificació/desnitrificació (N/DN) via nitrit i nitrificació parcial — Anammox (de l'anglès anaerobic ammonium oxidation). Ambdós possibilitats presenten un estalvi considerable en comparació amb la N/DN convencional. És ben sabut que la N/DN via nitrit suposa una reducció de les necessitats d'aireig i matèria orgànica. En el present treball s'ha estudiat la integració de l'eliminació de nitrogen via nitrit amb la producció de polihidroxialcanoats (PHA), bioplàstics d'alt interès comercial que s'acumulen a l'interior de les cèl•lules donades les condicions adients (sacietat — fam).

Tots els estudis s'han portat a terme a escala de laboratori i s'han obtingut conclusions satisfactòries en al majoria de casos, arribant a la conclusió que valoritzant la matèria orgànica present en les aigües residuals com una font de recursos i reduint el cost dels sistemes d'eliminació de nutrients, les EDAR esdevindran processos molt més sostenibles.