Strategies to enhance microalgae anaerobic digestion in wastewater treatment systems : pretreatments and co-digestion
Microalgae-based wastewater treatment systems are promising solutions to shift the paradigm from wastewater treatment to energy and resources recovery. In these systems, microalgae assimilate nutrients and produce oxygen which is used by bacteria to biodegrade organic matter improving water quality....
| Autor: | |
|---|---|
| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2018 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/663207 |
| Acceso en línea: | http://hdl.handle.net/10803/663207 https://dx.doi.org/10.5821/dissertation-2117-122707 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria civil 628 |
| Sumario: | Microalgae-based wastewater treatment systems are promising solutions to shift the paradigm from wastewater treatment to energy and resources recovery. In these systems, microalgae assimilate nutrients and produce oxygen which is used by bacteria to biodegrade organic matter improving water quality. Moreover, microalgae biomass can be harvested and reused to produce biofuels among other bioproducts. In this context, anaerobic digestion (AD) is one of the most consolidated and well-known technologies to convert organic waste generated in a wastewater treatment plant into bioenergy. However, microalgae AD is generally limited by their resistant cell wall, which lead to low methane potential (degradation extent) and conversion rate (degradation speed). Also, microalgae have high protein content, which can lead to ammonia nitrogen inhibition during the anaerobic digestion process. This PhD thesis aims to overcome these drawbacks and improve the technology by combining the use of pretreatments and the co-digestion. While pretreatments act disrupting or weakening the structure of microalgae cell wall, allowing the intracellular content to become more bioavailable, anaerobic co-digestion (i.e. the simultaneous digestion with two or more substrates) can contribute to improve microalgae AD performance by increasing methane potential, diluting inhibitory compounds or getting synergies between substrates (nutrients composition, rheology, etc.) in addition to the economic advantages derived from treating several wastes in a single facility. Firstly, co-digestion of harvested microalgae from high rate algal ponds (HRAP) used as secondary treatment for urban wastewater and primary sludge, which is produced in the same treatment process, is been investigated. Results have shown that the most suitable option to anaerobically digest microalgae from HRAPs would be the co-digestion with primary sludge at a 20-day hydraulic retention time (HRT), that leads to higher methane production (between 63% and 2.3-fold increase). The energy assessments conducted according to these results have revealed that microalgae co-digestion with primary sludge is a key technology for energy recovery in HRAPs, since the energy produced is up to 4-fold the energy consumed during the AD. Finally, potential reuse of microalgae digestates in agriculture has been investigated (including their co-digestion with primary sludge). all microalgae digestates have presented suitable properties for agricultural soils amendment, although digestate from co-digestion has presented the least phytotoxicity. Besides, co-digestion with storable agricultural wastes (i.e. wheat straw) is been evaluated. As it happens to microalgae, wheat straw AD is limited by hydrolysis step due to its lignocellulosic structure. Thus, their co-digestion with microalgae is also being investigated after a simultaneous thermo-alkaline pretreatment to both substrates. Results have shown that wheat straw co-digestion (50% VS) at 20-day HRT has increased microalgae methane yield by 77% as compared to microalgae mono-digestion (from 0.12 L CH4/g VS to 0.21 L CH4/gVS). On the other way around, pretreatment has only increased the methane yield by 15% as compared to untreated substrates co- digestion (0.24 L CH4/g VS). Thus, the co-digestion of microalgae and wheat straw is successful even without the pretreatment. Finally, when microalgae are used as tertiary treatment, waste activated sludge (WAS) results in abundant and available co-substrate. As a novelty, in this PhD thesis, microalgae and WAS co-digestion is investigated after applying a simultaneous autohydrolysis pretreatment at 55 °C to improve microalgae biodegradability by promoting inherent enzymes release from WAS. However, results have shown that WAS enzymes have not been effective at disrupting microalgae cell wall. Anyway, WAS co-digestion (80% VS) after pretreatment has increased microalgae mono-digestion methane yield up to 130%. |
|---|