Joint research project

Optimization of Anaerobic-Aerobic Sequential Process Applied to the Treatment of Sewage Sludge and Food Waste in Combination with Raw Wastewater

Project leaders
Mariaconcetta Tomei, Sophia Ghanimeh
LIBANO - CNRS-L - National Council for Scientific Research of Lebanon
CNR/CNRS_L 2015-2016
Earth system science and environmental technologies
Thematic area
Earth system science and environmental technologies
Status of the project

Research proposal

-Conventional treatment-
Traditionally, domestic wastewater is treated using aerobic biological processes, such as the Waste Activated Sludge (WAS) process, where aeration is provided to ensure adequate oxygen content for the growth of aerobic microorganisms. After removal of the relatively clean supernatant, the high-solids sludge, remaining at the bottom of the treatment units, is collected and sent for Anaerobic Digestion (AD). The latter consists of converting the biodegradable solids into CO2 and methane (CH4), a source of energy, by a consortium of anaerobic microorganisms. However, raw wastewater is seldom treated anaerobically, mainly due to its low-strength (low solids content) leading to low gas generation and low process efficiency.
Similarly, food waste is usually treated using aerobic biological methods, namely composting, where the organic matter in the waste is oxidized into CO2 and H2O. A considerable part of the inherent water (~70% of the food weight) is lost by evaporation or converted into a high strength leachate, which is difficult to treat and may end up in evaporation ponds where all the water is lost into the atmosphere.
In the last decade, there has been an increasing interest in applying AD technology to treat high-strength wastes such as food waste. Yet, given the high biodegradability of food waste, acids generation occurs, often fast, leading to pH drop and inhibition of the methane-generating microorganisms. Therefore, food waste is often diluted with water and digesters are fed at low organic rates (El Fadel et al. 2013, Ghanimeh et al. 2013 a, b and c, El-Fadel et al. 2012 a and b, Ghanimeh et al. 2012).

-Anaerobic-aerobic sequential systems (AASS) -Anaerobic treatment alone has generally not been sufficient to meet stringent effluent requirements for Chemical Oxygen Demand (COD) and suspended solids (SS), thus often necessitating a post-treatment (Kim et al., 2011). Recently, a novel system is being considered to improve the digestion of sewage sludge, consisting of an AD compartment followed by an aerobic reactor. The rationale for the additional aerobic step is that some of the organic fraction that cannot be anaerobically digested in the first compartment may be aerobically degraded in the second. Also, the aerobic stage is helpful in reducing the concentration of ammonia - a major concern for land application. In fact, it has been shown that the implementation of the aerobic step, in AASS, considerably reduces the nitrogen level in the effluent and improves the removal of volatile solids by 17% and COD by 21% (Tomei et al. 2011, Tomei and Carozza, 2014).
The proposed study consists of 2 experimental programs using the anaerobic-aerobic sequential system to: (1) improve sludge treatment efficiency; (2) achieve combined treatment of wastewater and food waste. The first experiment will be performed at IRSA-CNR, Italy, and the second will be performed at NDU, Lebanon. Experimental data will be utilized to formulate a kinetic process model. Also, the feasibility study of the novel system will be investigated. The results will be submitted for publication.
IRSA-CNR was extensively involved in the optimization of sludge treatment and modeling of the anaerobic process applied both to wastewater treatment and sludge stabilization. The Lebanese PI contributed to 2 previous CNRS projects on anaerobic digestion of waste and the results were published in international peer-reviewed journals and conferences (listed in the references list below).
- Significance and novelty - First, compared to AD alone, the combination of the two digestion phases can alleviate some of the drawbacks of each individual digestion method. Aerobic digestion is relatively simple and produces soil conditioner but is characterized by high energy consumption. In contrast, AD allows for energy recovery in the form of CH4 but is less stable and more complex. Moreover, the AASS is expected to improve the treatment efficiency and nitrogen removal from municipal wastewater sludge (Tomei et al. 2011), which can affect the next sludge treatment steps, limit the need for polymers and reduce the cost of sludge conditioning. To improve further the efficiency of the system, this proposal suggests applying mesophilic (35-37oC) conditions to the aerobic compartment - while most reported systems are operated at room temperature. This can be achieved with low energy input given the exothermicity of the aerobic process and the high temperature of the anaerobic effluent.
Second, compared to independent treatment of food waste (through composting) and wastewater (through WAS or AD of sludge), co-digestion in an AASS enhances the feasibility of wastewater and solid waste treatment. The need for a separate food waste composting facility is eliminated. Also, the generation of renewable energy (in form of CH4) is improved by adding high-strength food waste. On the other hand, compared to AD systems operated on food waste alone, the addition of wastewater is expected to improve the system stability by slowing down the generation of acids and providing continuous re-seeding with methanogens and other anaerobic microorganisms. Furthermore, compared to aerobic treatment or wet AD of food waste, no clean water is needed for dilution and all the moisture in wastewater and food waste can be recovered with minor losses leading to better water conservation.
Even though combined AD of food waste with wastewater solids (including primary and/or secondary sludge) has been addressed (Yu et al. 2014, Elbeshbishy et al. 2012), co-digestion with wastewater liquids (low solids content) in AD or sequential systems is lacking. This work can be useful in many developing countries, such as Lebanon, where a large number of wastewater treatment plants remain at the planning stage, thus allowing for potential modifications in design and feed type.

Research goals

The proposed project aims at applying the anaerobic-aerobic sequential system to:

1. Improve the treatability of domestic sludge, through modified operating temperature in the aerobic stage, resulting in low levels of solids, organics and ammonia in the effluent.

2. Investigate combined digestion (co-digestion) of wastewater with food waste, where the former provides the needed dilution effect for better system stability, and the latter ensures the high biogas generation rate for higher process efficiency.

3. Model the anaerobic-aerobic sequential system through kinetic test data analysis and fitting.

4. Assess the economic feasibility of anaerobic-aerobic sequential system compared to conventional treatment methods with special emphasis on small-scale applications.

Last update: 27/11/2021