Types of anaerobic reactors

The general types of anaerobic reactors which are relevant in this thesis are shortly presented here.

2.2.1 Continuous stirred tank reactor (CSTR)

This reactor is characterized by continuous and constant rates of both feeding and discharging.

The system has a complete mixing of substrate and bacteria. Both bacteria and wastewater have the same retention time, so hydraulic retention time (HRT) equals solid/sludge retention time (SRT). The term steady state can be applied to the CSTR under condition that the system is well adapted to the substrate and the loading is constant. Steady state conditions will ensure stable biogas production and output quality. This system was applied mostly for sewage sludge and manure digestion at mesophilic conditions. Benefits and drawbacks of CSTR are summarized in Table 6 (Fischer et al. 1986).

The CSTR system operates usually well at HRT above 15 or 20 days at mesophilic conditions. In general, it can be stated that mesophilic CSTR can be applied when the input is so concentrated as to at least provide enough biogas to produce the heating energy for the system (STOWA 2005). The higher the concentration of organic matter to be degraded, the more surplus energy is produced. Another important economic benefit is that the reactor volume becomes smaller assuming that the same HRT can be applied for a diluted and concentrated wastewater.

Low-tech systems operated at ambient temperature and fed discontinuously, e.g. daily, are also comprised under CSTR. They mostly have higher HRTs of up to 100 days.

Table 6: General benefits and drawbacks of continuous stirred tank reactors (CSTR) Benefits of CSTR Drawbacks of CSTR

High reliability Big reactor volumes as HRT = SRT Applicable for substrate with high suspended solid

concentration

Required energy for mixing

Good contact between substrate and micro-organisms

Possibility of undigested sludge substrate leaving the reactor

Homogeneous temperature throughout the tank Prevention of scum layer formation

The accumulation reactor (AC) is a variant of the CSTR, both systems being continuously fed.

While the output from the CSTR is continuously removed, the AC reactor is always filled until the effluent is only removed once, at the end of the filling period. The CSTR has a constant digestion volume while that of the AC system is increasing with time. The AC reactor has been applied for manure digestion (Wellinger and Kaufmann 1982) and for mixtures of manure and household waste and wastewater. The main benefit of AC reactor is the combination of digestion and storage, as in agricultural application, a storage tank is usually required to overcome the periods where organic fertilizer cannot be applied on the fields (El-Mashad 2003). In the last years, the application of the UASB septic tank as a combination of high-rate and accumulation reactor was investigated as a promising alternative.

2.2.2 Upflow anaerobic sludge blanket reactor (UASB reactor)

Several high-rate anaerobic systems were developed during the past decades, like the anaerobic filters (AF) (Young & McCarthy 1969), the upflow anaerobic sludge blanket (UASB) reactor (Lettinga et al. 1980) and the baffled reactor (Bachmann et al. 1985). The common feature between these reactors is that they operate at long SRT and short HRT. Amongst these systems, the UASB reactor is the most widely applied. The success of the UASB reactor can be attributed to its capability to retain high concentration of active suspended biomass with simple and low cost means. The formation of granular sludge which has a high methanogenic activity and is better settlable than flocculant sludge improves the maximum loading rate of the UASB system especially for high-strength wastewater with a high dissolved COD fraction. UASB reactors are also applied for domestic wastewaters; although no formation of granular sludge takes place, they are efficient at ambient and mesophilic temperature (Elmitwalli 2000, Wendland et al.

2007). The generated sludge within the UASB reactor is stabilized simultaneously and is discharged from time to time, depending on the suspended solid fraction in the influent

For onsite treatment of domestic wastewater, a variant of the UASB reactor, the so-called UASB-septic tank was first investigated by Bogte et al. (1993). The major difference in relation to the conventional UASB systems is that the UASB-septic tank is designed also to accumulate and stabilize the sludge. In relation to the conventional septic tank system, it differs by the up-flow mode implemented, achieving an improved suspended solid removal and better biological conversion (STOWA 2005). Bogte et al. (1993) reported a high removal efficiency at a temperature above 12 °C. Below 12 °C, the conversion of VFA to methane was too slow. In that case, Zeeman and Lettinga (1999) propose the pre-treatment in a UASB reactor to support hydrolysis.

2.2.3 Batch reactor

The batch reactor starts with a certain amount of inoculum and is filled with raw substrate. After the digestion, the reactor is emptied and refilled again. This reactor is very simple in design and in operation. Predominantly, batch reactors are applied to determine biodegradability at bench scale.

At technical scale batch reactors have been rarely applied e.g. for dry AD of solid wastes (Ten Brummeler 1993). Its operational drawbacks are firstly the accumulation of metabolites in the start-up phase and secondly great differences in the gas production rate over the digestion time.

To produce a more continuous biogas production, Zeeman (1991) proposed operating several batch digesters consecutively.