1. Introduction
Separation of domestic waste(water) at the source results in black water from the toilet (faeces and urine) and less polluted grey water from showers, laundry and kitchen. These source separated waste(water) streams differ in quantity and quality and should be treated separately according to their concentrations and composition. The main benefits of such an approach include the possibility of recovering energy and nutrients and the efficient removal of micro-pollutants. Grey water has a high potential of reuse because it is the major fraction (70%) of domestic wastewater and relatively low in pollution [
1]. Black water contains half the load of organic material in domestic wastewater, the major fraction of the nutrients nitrogen and phosphorus [
2,
3] and can be collected with a small amount of water (one liter per flush) using, for example, vacuum toilets. Black water also contains most of the pathogens, hormones and pharmaceutical residues. The volume of black water depends on the type of toilet and amount of water needed to flush.
Anaerobic treatment is regarded as the core technology for energy and nutrient recovery from source separated black water [
3,
4,
5] because it converts organic matter to methane, which can be used to produce electricity and heat, while at the same time anaerobic treatment yields low amounts of excess sludge. The nutrients are largely conserved in the liquid phase and can be subsequently recovered with physical-chemical processes such as precipitation and ion-exchange or removed biologically [
6,
7]. Depending on the distance to agricultural fields, direct reuse of nutrient rich anaerobic effluent is possible if it is treated to remove pathogens and micro-pollutants [
8,
9].
With an average load of 62 gCOD/p/d and a methanisation level of 60% [
10], 12.5 L CH
4/p/d can be produced from black water (0.35 L CH
4 /gCOD, (Standard temperature and pressure (STP))). When solid kitchen refuse is included ((60 gCOD/p/d), [
10]) the biogas production can be doubled, resulting in 25 L CH
4/p/d, which represents 335 MJ/p/y (35.6 MJ/Nm
3 CH
4). Combined heat and power (CHP) generation systems can be used to produce heat and electricity at an efficiency of 85% (of which 40% electricity and 60% heat) [
11]. This would result in a production of 32 kWh/p/y electricity (2.1% of the electricity consumption in a household (87 PJ electricity consumption in The Netherlands in 2006 [
12]
i.e., 1487 kWh/p/y)) and 47 kWh/p/y of heat when using the methane produced from black water and solid kitchen refuse.
The use of three types of reactors for anaerobic treatment of black water collected with vacuum toilets at different temperatures is reported in literature, namely a CSTR (continuously stirred tank reactor), an Accumulation system and a UASB-septic tank (Upflow anaerobic sludge blanket).
Wendland
et al. [
13] investigated anaerobic treatment of black water from vacuum toilets in a CSTR operated at mesophilic conditions (37 °C). A removal efficiency of total COD of 61% was achieved at an HRT (Hydraulic retention time) of 20 days. Applying a CSTR for anaerobic treatment of black water (7 L/p/d) requires a volume of 140 L per person [
13].
Kujawa-Roeleveld
et al. [
10] investigated anaerobic treatment of black water and kitchen refuse in an accumulation system operated at 20 °C. An accumulation system is a continuously fed reactor and combines digestion and storage in one reactor volume. Stabilization of the black water for 80% was achieved within 150 days. Due to the long storage time a relatively large volume is needed of 1.0 m
3 per person for the treatment of black water. An accumulation system therefore is only suitable for even more concentrated streams (e.g., only faeces (brown water) and kitchen waste) and less suitable for black water [
10].
The second system that Kujawa-Roeleveld
et al. [
10,
23] investigated was a UASB-septic tank operated at 15 and 25 °C. UASB reactors enable long sludge retention times (SRT) at relatively short hydraulic retention times (HRT), because biomass retention is accomplished by an internal gas/sludge/liquid separation system [
4]. A UASB-septic tank is a continuous reactor with respect to the liquid, but accumulates the solids, combining the features of a UASB reactor and a septic tank. The UASB-septic tank removed 61% of the total COD at 15 °C and 78% of the total COD at 25 °C. For sludge stabilization and total reduction of volatile fatty acids (VFA) at 25 °C a minimum volume of 200 L per person is needed, corresponding to an HRT of about 30 days [
10].
The reactors mentioned above require relatively large volumes per person (
Table 1). Unlike the UASB-septic tank, a UASB reactor without additional space for the accumulation of solids (no septic tank) would require regular sludge removal, but it will reduce the volume of the reactor [
4]. This is important for application at larger scale where space might be limited.
Table 1.
Reactors for anaerobic treatment of concentrated black water.
Table 1.
Reactors for anaerobic treatment of concentrated black water.
| CSTR [13] | Accumulation system [10] | UASB-septic tank [10,23] |
---|
Temperature (°C) | 37 | 20 | 15 | 25 |
Total COD removal (%) | 61 | 80 | 61 | 78 |
HRT (d) | 20 | 150 | 30 | 30 |
SRT (d) | 20 | 150 | >365 | >365 |
Volume required (L/p) | 140 | 1.0*103 | n.d. | 200 |
Methanisation* (%) | 60 | 58 | 39 | 60 |
UASB reactors so far have not been investigated for their capability to treat concentrated wastewater streams such as black water and was only shortly discussed by Zeeman
et al. [
14]. The volume of a UASB reactor will depend on the minimum SRT required to achieve methanisation and stabilization of the sludge [
4]. For the anaerobic treatment of black water hydrolysis of particulate organic substrates is the rate-limiting step [
15]. With first order kinetics and a hydrolysis constant of 0.1 d
-1 (average value at 20–30 °C [
16]) it can be calculated that a high percentage of hydrolysis (between 80 and 90%) can be achieved at a SRT between 40 and 90 days. Other research showed as well that the minimum SRT was estimated to be 75 days at 25 °C to achieve methanisation and stabilization of the sludge [
4,
17]. Other factors that are important for the anaerobic treatment of black water are the temperature and inhibition by free ammonia [
18]. Luostarinen
et al. [
19] investigated the effect of temperature on anaerobic treatment of black water in UASB-septic tanks. The temperature had no significant effect on suspended solids removal, but the removal of dissolved COD improved because sludge adapted to lower temperatures (15 °C) [
19]. The black water can be produced at a temperature of about 20 °C [
20]. A higher temperature could result in a shorter HRT, but this would require extra energy requirements for heating the black water. Therefore a temperature of 25 °C was selected for the treatment of black water in a UASB reactor. In concentrated black water high concentrations of ammonium (0.8–1.4 gNH
4-N/L) are present which can inhibit methanisation and therefore higher retention times could be needed to achieve a maximum production of methane [
21].
This paper describes the feasibility of applying a compact UASB reactor for the treatment of concentrated black water from vacuum toilets at these conditions. Furthermore the design of the UASB reactor will be discussed, as well as the minimum volume needed at full scale.