Abstract:
The current disposal of sludge from wastewater treatment of fish processing plants operating in Portugal is the landfill, a costly management solution which is under restriction, according to policy on organic waste disposal (Landfill Directive). In this purpose, the aim of this work was to assess the potential use of this substrate for the production of biogas through anaerobic digestion, a long proven and efficient way for the production of a renewable fuel – Biogas. This renewable energy resource can be used as a source of energy to produce electricity and heat, reducing consequently the processing costs and, in addition, the industry carbon footprint. So, the produced sludge can become a valuable sub-product of the wastewater treatment process, fulfilling the directive target and avoiding the associated disposal costs. This study looks at achieving reliable data on biogas production and degradation yield. Laboratory batch assays were performed at mesophilic temperature (35±1°C) during 52 days. The anaerobic digestion of the fish sludge performed efficiently and showed a high methane production yield, corresponding to 700 m3 of CH4/ton VS, which is a really promising result, in terms of energy benefits.
Key words
Anaerobic digestion, IWWTP sludge, biogas, fish industry.
Introduction
Environment and energy production are two distinct areas that are getting closer as the world population grows and the fossil fuels become even more a national security and environmental concern. Climate change and petroleum dependence boosted the demand for new energy sources, cleaner and from affordable raw materials. The world growing population and the increasing western lifestyle habits lead to a boost in the amount of waste generated, which causes handling and environmental problems. To overcome these situations in a sustainable manner there is a worldwide growing concept which promotes the production of energy from different wastes (Waste – to – Energy), derived from agricultural, industrial and/or domestic sources (Kothari et al., 2010).
Energy can be recovered from waste in distinct ways, depending on the technology used, being incineration currently the most popular, especially in countries with district heating coverage. However, there are other options available which use the biodegradable fraction of the waste to the production of bioethanol, biodiesel or biogas. These types of biofuels can be used not only to produce electricity and heat but also fuel for transportation. A study conducted by Münster and Lund (2010) comparing different technologies suggests that anaerobic digestion (AD) is one of the best alternative technologies to the energy production from the waste organic fraction.
AD process produces two distinct products, biogas and digestate. Biogas is mainly composed by methane (CH4) and carbon dioxide (CO2) and can be used in CHP engines, microgas turbines or in fuel cells. It can also be injected into the natural gas grid and be used in different gas applications, as for example vehicle fuel, although it needs to be cleaned before being injected to reach the quality standards defined by countries specific regulations. The digestate is the liquid fraction resulting from the AD process and it can be used as organic fertilizer (Weiland, 2010), saving not only the resources needed to produce chemical fertilizers as also the GHG emissions resulting from its production. This two products resulting from the AD process can be sell or used within the industrial process (biogas), reducing the overall plant processing costs and the plant carbon footprint, due to the utilization of a renewable fuel resource.
In Portugal few industrial wastewater treatment plants (IWWTP) are using AD as biological treatment, ending-up the process with a sludge rich in organic compounds that normally is sent to landfill. In this work we used a sludge from a fish canning industry, due to its potential use as AD substrate and also because among all the different food sectors fisheries are one of the fastest growing in the world, with a global total production continuously increasing and reaching approximately 140 million tonnes in 2007 (Figure 1). About 81% of total fishery production (114 million tonnes) was used for direct human consumption, with 15% (17 million tonnes) of it being processed by the canning industry. The non-food utilization of the fishery products is mainly for the manufacture of fishmeal and fish oil. World average per capita supply of fish grew from 9 kg in 1961 to 17,1 kg in 2007, with fish accounting for 16,1% of the global population’s intake of animal proteins and 6,2% of all proteins consumed (FAO 2010). This worldwide growing industry is very attractive in terms of organic waste production and its potential utilization in the generation of renewable energy.
The reported work on international literature about AD applied to the fisheries industry refers only to the treatment of wastewater, there is nothing previously done using mixed sludge from primary and secondary the wastewater treatment (Mendez et al. 1992; Puñal and Lema 1999; Palenzuela-Rollon et al. 2002 and Chowdhury et al. 2010). The main objective of the present work was to assess the methane yield obtained from the AD of IWWTP sludge working in a fish canning industry, in batch experiments under mesophilic conditions, using as inoculum digested sludge from a mesophilic anaerobic digester operating in a municipal WWTP from Lisbon (Chelas).
