High-rate anaerobic wastewater treatment - Water Science ...

Q IWA Publishing 2008 Water Science & Technology—WST | 57.8 | 2008

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High-rate anaerobic wastewater treatment: diversifying from end-of-the-pipe treatment to resource-oriented conversion techniques Jules B. van Lier

ABSTRACT Decades of developments and implementations in the field of high-rate anaerobic wastewater treatment have put the technology at a competitive level. With respect to sustainability and costeffectiveness, anaerobic treatment has a much better score than many alternatives. Particularly,

Jules B. van Lier Sub-department of Environmental Technology, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands

the energy conservation aspect, i.e. avoiding the loss of energy for destruction of organic matter, while energy is reclaimed from the organic waste constituents in the form of biogas, was an important driver in the development of such systems. Invoked by the present greenhouse alert,

Lettinga Associates Foundation (LeAF), P.O. Box 500, 6700 AM Wageningen, The Netherlands E-mail: [email protected]

the energy involved is nowadays translated into carbon credits, providing another incentive to further implement anaerobic technology. Anaerobic conversion processes, however, offer much more than cost-effective treatment systems. Selective recovery of metals, effective desulphurization, recovery of nutrients, reductive detoxification, and anaerobic oxidation of specific compounds are examples of the potentials of anaerobic treatment. This paper presents a survey on the state of the art of full-scale anaerobic high-rate treatment of industrial wastewaters and highlights current trends in anaerobic developments. Key words

| anaerobic treatment, energy, resource recovery

INTRODUCTION Anaerobic wastewater treatment (AnWT) has evolved into a

3.

Many different types of organically polluted wastewaters, even those that were previously believed not to be suitable

reactor volumes 4.

for AnWT, are now treated by anaerobic high-rate conversion processes. In countries like The Netherlands, almost all

High applicable COD loading rates reaching 20 –35 kg COD.m23 reactor volume.day21, requiring smaller

competitive treatment technology in the past few decades.

No use of fossil fuels for treatment, saving about 1 kWh/kg COD removed

5.

Production of about 13.5 MJ CH4 energy/kg COD

agro-industrial wastewaters are presently treated with

removed, giving 1.5 kWh electric output (assuming

anaerobic reactor systems. Analysing the reasons why the

40% electric conversion efficiency).

selection for AnWT was made, the following striking

6.

advantages of AnWT over conventional aerobic treatment systems can be given: 1. 2.

Reduction of excess sludge production up to 90%

Rapid start-up (, 1 week), using granular anaerobic sludge as seed material

7.

No or very little use of chemicals

8.

Plain technology with high treatment efficiencies

9.

Anaerobic sludge can be stored unfed; reactors can be

Up to 90% reduction in space requirement when using

operated during agricultural campaigns only (e.g. 4

expanded sludge bed systems

months per year in the sugar industry)

doi: 10.2166/wst.2008.040

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J. B. van Lier | High-rate anaerobic wastewater treatment

10. Excess sludge has a market value 11. High-rate systems facilitate water recycling in factories

Water Science & Technology—WST | 57.8 | 2008

plant, the generation of 1 MW of electricity emits about 20 ton CO2/day, estimated based on the following assump-

(towards closed loops)

tions: coal power plant electric conversion efficiency:

Obviously, the exact ranking of the above advantages

37.5%; CO2 in off-gas: 15.3%; off-gas emission: 7.1 Nm3/kg

depends on the local economic and societal conditions. In the Netherlands, excess sludge handling is the costdetermining factor in operating wastewater treatment systems. Since landfilling is not an option for excess sewage sludge and biowastes, while prices for incineration reach e450– 500/ton wet sludge, the low sludge production in anaerobic reactors is an immediate economic benefit. The system compactness, another important asset of AnWT, can be illustrated by a full-scale example, where an anaerobic reactor with a 6 m diameter and a height of 25 m, suffices to treat up to 25 tons of COD daily. The produced sludge, which is less than 1 ton dry matter/day in this example, is not a waste product, but is marketed as seed sludge for new reactors. Such compactness makes the system suitable for implementation on industrial premises or sometimes even inside the factory buildings. The latter is of particular

of coal; coal ash-content: 10%; energy content of coal: 25 MJ/kg coal. At a foreseen stabilised price of e 20/ton CO2, the abovementioned industrial application could earn e 500/day on carbon credits, while no fossil fuels are used for treating the wastewater. Although this amount is negligible in industrialised countries, it could provide a real incentive in developing countries to start treating the wastewater using high-rate AnWT, and thereby protecting the local environment. The carbon credit policy can, therefore, be regarded as a Western subsidy for implementing AnWT systems in the less prosperous countries. Obviously, in such applications, a potential leakage of CH4 to the atmosphere should be avoided as it rapidly offsets the benefits of reduced CO2 emissions, considering the CH4 greenhouse warming potential (GWP) being 23 times the GWP of CO2.

interest in densely populated areas and for those industries aiming to use anaerobic treatment as the first step in a treatment for reclaiming process water. The renewed interest in the energy aspects of AnWT

ANAEROBIC WASTEWATER TREATMENT The breakthrough

for industrial AnWT applications

directly results from the ever rising energy prices and the

occurred in the mid-seventies/eighties of the last century,

overall concern on global warming. The above 25 tons

following the development of anaerobic sludge bed tech-

COD/day of agro-industrial waste(water) can be converted

nology at lab and pilot scale. Figure 1 shows the gradual

in 7,000 m3 methane/day (assuming 80% CH4 recovery

increase in number of anaerobic high-rate reactors from the

based on average full-scale treatment efficiencies), with an

mid-seventies onwards. At present, a total number of 2266

energy equivalent of about 250 GJ/day. Working with a

registered full-scale installations are in operation, which are

modern combined heat and power (CHP) gas engine

constructed by renowned companies like Paques, Biothane,

reaching 40% efficiency, a useful 1.2 MW electric power

Biotim, Enviroasia, ADI, Waterleau, Kurita, Degremont,

output can be achieved. The overall energy recovery could

Envirochemie, GWE, Grontmij as well as other local

even be higher (reaching up to 60%) if all the excess heat

companies. To this number an estimated 500 ‘homemade’

can be used on the factory premises or direct vicinity.

reactors can be added which are constructed by very small

Assuming that full aerobic treatment would require

local companies or by the industries themselves but which

^1 kWh/kg COD removed, or 1 MW installed electric

do not appear in the statistics.

power in the above case, the total energy benefit of using

Key to this success of AnWT is the development of

AnWT over the activated sludge process is 2.2 MW. At an

high-rate

energy price of 0.1 e/kWh this equals about 5,000 e/day.

uncoupling of the solids retention time from the hydraulic

reactor

systems,

allowing

for

an

extreme

Apart from the energy itself, current drivers include the

retention time. This uncoupling can be achieved by

carbon credits that can be obtained by generating renewable

various ways of sludge retention, such as sedimentation,

energy using AnWT. For an average coal-driven power

immobilization on a fixed matrix or moving carrier


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Figure 1

|


Increase in installed number of anaerobic high-rate reactors, period 1972–2007 ( p incomplete data in 2007).

material, and granulation. High-rate systems can be

technology for industrial wastewater. After the initial first

divided into suspended growth and attached-growth

trials in the seventies, the system rapidly became popular,

processes including expanded/fluidized bed reactors and

particularly in the agro-food sector. The worldwide applied

fixed-film processes. In suspended growth systems bac-

technologies, implemented between 1980 and 2007 are

terial sludge is present as flocs or granules, whereas in

depicted in Figure 3 (left).

attached growth systems micro-organisms are adhered to

The right-hand graph shows the relative number of the

a moving or fixed medium. In an expanded/fluidized bed

same technologies in the time frame 2002 – 2007, with a

reactor, suspended carrier media, such as sand or porous

total number of 610 installed reactors. Figure 3 indicates

inorganic particles, are used to develop an attached film.

that the granular sludge based technologies (UASB, ICw,

Alternatively, expanded bed reactors, such as EGSB and

EGSB) dominated the market in the past decades. This is

w

(see below), are seeded with excess granular sludge

confirmed by the newly installed systems in the most recent

coming from other high-rate AnWT systems. Fixed film

period. Interestingly, competitive technologies like anaero-

processes rely on the bacteria attaching to fixed media,

bic filters or hybrid systems were not able to be consolidat-

like rocks, plastic rings, modular cross-flow media, etc.

ing in the market. But also the advanced Fluidized Bed (FB)

Some systems, such as the anaerobic hybrid process,

high rate technology almost vanished, most likely due to

combine suspended- and attached-growth processes in a

technology problems in various full scale systems. An

single reactor to utilize the advantages of both types of

interesting observation in Figure 3 is the increasing

biomass. Figure 2 compares the relative loading capacities

popularity of the expanded bed reactors EGSB and ICw.

of several anaerobic systems, emphasizing the need for

At present the major Dutch constructors (Paques and

efficient retention of active bacterial mass and the

Biothane) sell more ICw and EGSB reactors than conven-

required good contact between the wastewaters and the

tional UASB systems (Figure 4). Most likely, the vast

sludge for reducing the mass transfer resistances.

growing experiences and the higher availability of the

IC

(UASB)

indispensable seed material for these systems, i.e. methano-

reactor technology is considered the breakthrough in

genic granular sludge, have led to the success of the

the development and application of anaerobic high-rate

granular sludge based expanded bed systems.

The

upflow

anaerobic

sludge

blanket


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Figure 2

|


Relative loading capacity of different AnWT systems. Maximum applied loading rates under full scale conditions reach about 45 kg COD/m3.day using EGSB type systems.

In addition to the anaerobic reactor technology itself,

single step (Biothane 2006, personal communication). Even

there is increasing experience in pre- and post treatment

with relatively simple wastewater flows, like those coming

systems, safeguarding stable operation and guaranteeing the

from the beer brewery process, adequate pre- and post-

contracted effluent discharge criteria. Effluents containing

treatment is essential for the success of the system. In some

fatty, oily and greasy compounds (FOG), such as dairy

cases the actual anaerobic reactor volume is only 20– 25%

wastewater, are in some cases pre-treated to such extent

of the totally installed wastewater treatment volume for

that all FOG and suspended solids are removed from the

brewery effluents (European Brewery Convention 2003).

wastewater prior to feeding it to the high-rate system.

Novel developments in anaerobic reactor technology are

However, in other situations, the resulting treatment train

directed to integrated multifunctional bioreactors, such as

becomes so complex that decisions are made to apply

sequencing batch reactors (SBR), integrated anaerobic-

conventional CSTR systems that treat the entire flow in a

aerobic systems and a simplification of the treatment trains.

Figure 3

|

Implemented anaerobic technologies for industrial wastewater pictured for the period 1981– 2007 (left) and the period 2002–2007 (right). UASB: upflow anaerobic sludge blanket; EGSB: expanded granular sludge bed; ICw: internal circulation reactor; type of EGSB system with biogas-driven hydrodynamics; AF: anaerobic filter; CSTR: continuous stirred tank reactor; Lag.: anaerobic lagoon; Hybr.: combined hybrid system with sludge bed at the bottom section and a filter in top; FB: fluidized bed reactor.


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|

Figure 4


Relative number of installed UASB and Expanded Bed reactors, period 1984–2007.

End-of-the-pipe-treatment Most applications of AnWT can be found as end-of-the-pipe treatment technology for food processing wastewaters and agro-industrial wastewater. Table 1 lists the various industrial sectors where the surveyed 2266 reactors are installed.

compounds, such as poly chloro-aromatics and poly nitroaromatics as well as the azo-dye linkages can only be degraded when a reducing (anaerobic) step is introduced in the treatment line. Anaerobics are then complementary to aerobics for achieving full treatment. Based on the numerous laboratory researches that

It should be noticed that the number of anaerobic

were conducted in the past decades it is expected that the

applications in the non-food sector is rapidly growing.

AnWT

Common examples are paper mills and chemical waste-

Research with bench-scale and pilot scale reactor systems

waters, such as those containing formaldehyde, benzal-

has demonstrated that AnWT is applicable in a very wide

dehydes, terephthalates, etc. (Razo-Flores et al. 2006). The

temperature range, i.e. between 10 and 808C (e.g. Van

application

potential

will

steadily

increase.

latter is surprising since particularly the chemical industries

Lier et al. 1997), whereas COD concentrations as low as

are difficult to enter with anaerobic technology, owing to

100– 200 mg.l21 (e.g. Kato et al. 1994) and as high as

the general prejudices against biological treatment and

100,000 mg.l21 can be applied. Also the impact of toxic

anaerobic treatment in particular. With regard to the

compounds is much better understood and corrective

chemical compounds it is of interest to mention that certain

process engineering measures are described in the literature.

Table 1

|

Application of anaerobic technology to industrial wastewater. Total number of registered installed reactors ¼ 2,266 census January 2007

Number of Industrial sector

Type of wastewater

reactors

%

Agro-food industry

Sugar, potato, starch, yeast, pectin, citric acid, cannery, confectionery, fruit, vegetables, dairy, bakery

816

36

Beverage

Beer, malting, soft drinks, wine, fruit juices, coffee

657

29

Alcohol distillery

Can juice, cane molasses, beet molasses, grape wine, grain, fruit

227

10

Pulp & paper industry

Recycle paper, mechanical pulp, NSSC, sulphite pulp, straw, bagasse

249

11

Miscellaneous

Chemical, pharmaceutical, sludge liquor, landfill leachate, acid mine water, municipal sewage

317

14


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Obviously, each specific condition requires its own process

for beet transport, washing, and chipping is, after grit and

technology and reactor hardware. However, putting novel

sand removal, directed to the AnWT unit, and after aerobic

systems into practice simply needs time, while they need a

polishing, reused as process water for transporting, washing,

demand-market of interesting size. It must be noted that

and chipping again. In a modern beet sugar mill the

successful UASB trials at demonstration and full-scale were

mentioned process water loop may reach flows of

already accomplished in the late seventies, but it took over

200 m3/h or even more. Land water storage and concomi-

15 years before the technology was accepted as a feasible

tant settling of fine clay particles occurs in large storage or

alternative (Figure 1). And still, worldwide, many food

settling ponds in a water loop connected to the previously

processing industries opt for activated sludge processes

described intense loop. Any (soluble) organic matter

because of the relative novelty of anaerobic high-rate

present will decompose in these ponds, leading to odour

systems.

problems in the vicinity, owing to volatilisation of produced fatty acids. Interestingly, the AnWT plant (at present renamed as energy and water recovery unit) produces

Process water recycling towards zero-effluent-

sufficient alkalinity to avoid CaO additions, which were

discharge

previously required as neutralising agents in the mentioned

Reduction in industrial water consumption is generally

large storage/settling ponds. The overflow of the process

achieved by good housekeeping and redesigning the process

water loop is combined with the sugar refinery wastewater

water loops. Benefits to the companies include cost savings

and subsequently post-treated for nitrogen removal. The

resulting from e.g. less tax for water intake, less energy

energy benefit of the ‘closed water loop approach’ distinctly

consumption, less losses of raw materials, and less costs for

increases when the energy content of the treated warm

wastewater treatment since smaller units can be designed.

effluent is needed in the production process. An interesting

Also environmental improvement (green label), increased

example is the pulp and paper industry where ‘zero-effluent-

throughput, and risk and liability reduction may result from

discharge’ already can be achieved in the cardboard and

optimized process water cycles. In clean industrial

packaging paper manufacturers (Figure 5). Compared to an

production processes, water use reduction is essential.

open system that consumes 10 m3 of water per ton of paper

After good housekeeping, a generally warm and more

produced, the closed system saves 1045 MJ.ton21, simply by

concentrated process water stream is left that can be more

avoiding steam injection for raising the temperature of fresh

easily treated by anaerobic high-rate reactors. The oldest

water, assuming an intake water temperature of 108C and

industrial example where AnWT is part of the process water

an effluent of 358C. The surplus energy of the anaerobic

recovery loop is the sugar beet processing factory. The water

system adds another 200 MJ.ton21 paper, considerably

Figure 5

|

‘Zero-discharge’ cardboard and packaging industry of the Smurfit Kappa Group in Zu¨lpich, Germany (after Habets & Knelissen 1997).

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reducing the energy costs. In this case, the role of the

1995). Under such conditions, membrane assisted retention

anaerobic reactor is more than a treatment system: its cost-

of active methanogenic biomass would be of interest.

effectiveness leads to a more rapid implementation of the

Anaerobic membrane bioreactors (AMBRs) are also of

zero-discharge approach with all the benefits for the

interest for retaining specifically required, slow-growing

industry. With regard to the closed paper mill, it must be

micro-organisms for performing a key conversion in the

noted that in addition to COD removal, the anaerobic

overall degradation pathway. The process feasibility of

reactor also eliminates sulphate as sulphide from the

AMBRs has been recently demonstrated by various authors,

process water cycle, reducing the smell inside the factory.

although the economic feasibility is still questionable, owing

In fact, the anaerobic reactor is the only place for an

to the low achievable fluxes that highly depend on cake-

efficient sulphur bleed at zero costs. If biogas desulphurisa-

layer deposition (Jeison & van Lier 2006).

tion is required, the produced sulphide, which is stripped with the biogas, can be converted into elemental sulphur by applying micro-aerobic sulphide oxidation techniques

Anaerobic treatment of municipal sewage

(Lens & Hulshoff Pol 2000). Although not yet feasible in many industries, the zero-

Municipal wastewater is in quantity the most abundant type

discharge approach is now being researched for various

of wastewater on earth and is generally characterised by

types of industries, e.g. white paper, textile, and chemical

COD concentrations between 400 – 1,000 mg.l21. Under

industries. The next step in process water recycling will be

(sub)tropical climate conditions, municipal wastewaters

the agro-industrial production lines that are subjected to

reach temperatures ideal for AnWT (Van Haandel &

more stringent hygienic standards. However, reuse of

Lettinga 1994; Von Sperling & Chernicharo 2005). The

treated water for low-grade applications, such as washing

UASB reactor presently is the most frequently applied

and transportation has been applied for several decades as

system. Since the early nineties, hundreds of full scale

mentioned above.

UASB reactors have been constructed from 50– 50,000 m3

The closure of process water loops changes the

in volume. Generally, a biological oxygen demand (BOD)

characteristics of the resulting wastewater stream drasti-

reduction between 75 and 85% is realised, with effluent

cally. In addition to the heat conservation and increase in

BOD concentrations of less than 40 – 50 mg.l21. Total

COD concentrations, also potential inhibiting and recalci-

removal rates with regard to COD and total suspended

trant compounds may increase, affecting the anaerobic

solids (TSS) are up to 70 – 80% and sometimes even higher

treatability of the process water. To date, researches are

(Van Haandel & Lettinga 1994; von Sperling & Cherni-

oriented to understand the inhibition mechanisms at the

charo 2005). The required reactor systems are relatively

various trophic levels. Obviously, a better insight in toxicity

plain and, compared to activate sludge systems, require

and recalcitrance in anaerobic conversions will also

distinctly less functional units, making application at any

immediately amplify the action radius of AD technology,

scale very attractive. In fact, a single step UASB reactor

for instance to the above mentioned chemical industries. A

comprises 4 functional units, i.e. i) the primary clarifier, ii)

high degree of ‘process water loop closure’ may eventually

the biological reactor (secondary treatment), iii) the

lead to very extreme conditions with wastewater character-

secondary clarifier and iv) the sludge digester (Van Lier &

istics beyond the known operational conditions for existing

Huibers 2004).

AnWT systems. With the raise of temperatures and salinity

In order to comply with local regulations for discharge,

to extreme values, the performance of the commonly

the UASB system is generally accompanied by a proper

applied sludge bed systems (Figure 4) cannot be guaranteed.

post-treatment system, such as: facultative ponds, sand

Many authors already described the negative impact of

filtration, constructed wetlands, trickling filters, physico-

(extreme) thermophilic conditions and high salt concen-

chemical treatment, and activated sludge treatment (von

trations on the stability of methanogenic granular sludges

Sperling & Chernicharo 2005). The UASB reactor and the

and biofilms (e.g. Uemura & Harada 1993; Mendez & Lema

post-treatment step can be implemented consecutively or in


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Table 2

|


Main features and constraints of anaerobic sewage treatment in anaerobic high rate systems

Advantages (compared to activated sludge processes) † Substantial savings, reaching 90%, in operational costs as no energy is required for aeration † 40– 60% reduction in investment cost as less treatment units are required † If implemented at appropriate scale, the produced CH4 is of interest for energy recovery and/or electricity production † The technologies do not make use of high-tech equipment, except for main headwork pumps and fine screens. The treatment system is less dependent from imported technologies † The process is robust and can handle periodic high hydraulic and organic loading rates † Technologies are compact with average HRTs between 6 and 9 h and are, therefore, suitable for application in the urban areas, minimising conveyance costs † Small scale applications allow decentralisation in treatment, making sewage treatment less dependent from the extent of the sewerage networks † The excess sludge production is low, well stabilized and easily dewatered so does not require extensive post treatment † The valuable nutrients (N and P) are conserved which give high potential for crop irrigation † A well designed UASB filters helminth eggs from the influent, a prerequisite prior to agricultural reuse Constraints † Anaerobic treatment is a partial treatment, requiring post-treatment for meeting the discharge or reuse criteria † The produced CH4 is largely dissolved in the effluent (depending on the influent COD concentration). So far no measures are taken to prevent CH4 escaping to the atmosphere † The collected CH4 is often not recovered nor flared † There is little experience with full-scale application at moderate to low temperatures † Reduced gases like H2S, that are dissolved in the effluent may escape causing odour problems.

a more integrated set-up. The main features and constraints of anaerobic sewage treatment are listed in Table 2.

Novel developments in anaerobic sewage treatment include applications to concentrated wastewaters as can be

During the early developments of anaerobic sewage

found in The Middle East, Africa and the Arabic peninsula.

treatment some of the constraints were simply ignored or not

In Jordan and Palestine, municipal sewage reaches COD

taken into consideration in the full scale design because of

concentrations of 2,500 mg COD.l21 at TSS/COD ratio’s of

financial limitation. This however, results in negative experi-

0.6 (Mahmoud et al. 2004), whereas winter temperatures

ences and bad advertisement. Nowadays, uncontrolled CH4

may drop to 158C. Applying the conventional UASB reactor

emissions should be avoided and non-flaring of captured CH4

design, the hydraulic retention time (HRT) needs to be

should be prohibited. If instead all the energy is used, with the

increased reaching values of 20– 24 hours (Hallalsheh et al.

increasing energy prices and tradable CO2 credits (see above),

2005). This, obviously, will affect the hydrodynamics of the

anaerobic sewage treatment may even become an affordable

system, requesting changes in influent distribution for

investment for many developing countries. For most of the

preventing short-circuiting. Alternatively, the large sus-

listed constraints technical solutions are available, or at least in

pended solids load can be addressed in separate reactor

development. For example, the recovery of the methane from

units such as a primary clarifier or enhanced solids removal

effluents seems feasible applying low pressure suction after

in upflow filter systems, coupled to a sludge digester. A

which the exhaust air is subsequently directed to the flare or

novel approach is to link the UASB reactor to a coupled

the furnace as burning air for the captured CH4. With all

digester with sludge exchange (Mahmoud et al. 2004). With

constraints addressed, anaerobic sewage treatment has very

the latter system, accumulating solids will be digested at

big potentials to solve the major wastewater related problems

higher temperatures, whereas the methanogenic activity in

in developing countries.

the reactor will be increased by a return digested sludge

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flow. Particularly for the arid cimate zone, AnWT may

conversion process is that the original biomass energy

represent the first crucial step in reclaiming the secure

content is hardly altered! The amount of energy enclosed in

urban water sources for agricultural production, effectively

1 kg COD equals about 13.5 MJ. Whether this energy is

using the solubilised nutrients (Van Lier & Huibers 2004,

captured in gaseous form as CH4, H2, or other reduced

2007). The recovered energy can be beneficially used for

gases, or as liquid compounds such as alcohols, depends on

upgrading the pre-treated sewage to reuse standards and/or

to what extent the anaerobic conversion process is allowed

to operate irrigation works. Based on pilot trials in Amman,

to proceed. The most traditional energy recovery process is

3

the potential CH4 production equals 27,000 m .d

21

for a

alcohol fermentation, which currently finds its new revival

daily flow of 180,000 m3.d21 at an average COD concen-

in the production of renewable biofuels from sugar-rich

tration of 1500 mg.l

21

, equivalent to a potential power

substrates. Brazil is the leading country in sugarcane

supply of < 5 MW-e (assuming 40% CHP efficiency). In this

fermentation for alcohol fuel production, although the

3

21

calculation a modest CH4 recovery of 0.15 Nm CH4 kg

energy balances in these industries are far from optimised

COD removed, has been considered.

(Van Haandel 2005). The high interest in CH4 as energy-rich

Increasing energy prices and present concern on fossil

end-product started in the mid-seventies leading to a wide

fuel consumption, gives ample potential to anaerobic

range of manure, slurry and solid waste digesters as well as

sewage treatment for offering a feasible alternative for

anaerobic wastewater treatment technologies as outlined in

treating the huge flow of domestic and municipal waste-

the previous section. At present, countries like Germany

waters in many parts of the world. In light of the current

and Austria heavily promote centralised anaerobic digesters

green house gas and energy discussion, recovery of all

for the production of renewable energy from energy crops

produced CH4 should be an intrinsic part of the treatment

and agrowastes. Electricity generation using the generated

plant design. Owing to its compactness, high-rate anaerobic

CH4 is subsidised with ‘green funds’ reaching values of

sewage treatment can be applied in the urban areas as well.

e 0.20 per kWh. A more novel energy carrier is H2, a clean

The latter will lead to huge costs reduction in constructing

carbon free energy rich gas, which is often mentioned to

sewerage networks, pumping stations, and conveyance

play an important role in the future hydrogen based

networks. It must be realised that only 35% of the produced

economy. Current research on hydrogen fermentation

municipal wastewaters in Asia are treated, whereas in Latin

focuses on culture description, privileged substrates, factors

America this value is only 15% (WHO/Unicef 2000). In

affecting microbial conversions, and optimising reaction

Africa, the generated wastewaters are hardly collected and

kinetics. Albeit lab and modest pilot trials are very

sewage treatment, with the exception the Mediterranean

promising, large scale hydrogen fermenters are not yet

part and South Africa, is nearly absent.

constructed. The most recent development is to directly scavenge the anaerobically liberated electrons at an anode, which, in combination with a cathode under oxidising

RESOURCE RECOVERY USING AD TECHNOLOGIES Energy

conditions creates an electric current for immediate use (Logan et al. 2006). The fact that no further conversion step for electricity generation is required, creates possibilities for

During the oxidation of organic matter electrons are

decentralised electricity production from organic matter

channelled from carbon atoms to an electron acceptor. If

even at very small scales.

present, O2 would scavenge all energy-rich electrons leading to a complete loss of the biomass energy in low value heat. Under anaerobic conditions carbon atoms themselves are the main electron scavengers leading to a

Metals

pool of carbon products with either a high oxidation state

Alternative to organic carbon, oxidised sulphur compounds,

(CO2) or a very low one, such as the gaseous CH4 or the

22 22 such as SO22 4 , SO3 , and S2 O3 , can also act as electron

liquid CH3CH2OH. Most important in the anaerobic

scavenger under anaerobic conditions leading to a complete

1146



reduction of these compounds to sulphide. In fact, the

† The applicable pH range where almost complete pre-

thermodynamics of sulphate reduction are very competitive

cipitation is possible is much wider compared to MeOH

to methane formation, while similar substrates are used by

ranging from 2 – 12 depending on the type of MeS.

the sulphate reducing bacteria (SRB), employing similar reaction kinetics. For this reason, oxidised sulphur com2

† MeS precipitation is less susceptible for chelating compounds, leading to easy recoverable sludges.

pounds will always be reduced to HS , when exposed to

Metal recovery using anaerobic S technologies is

anaerobic conditions in the presence of an electron donor.

extensively researched and pilot and full scale examples,

Generally regarded as a nuisance, since it smells, reduces

e.g. for the treatment of acid mine drainage, are found on

the CH4 production rate, is toxic for methanogens, and is

several locations. An overview of the state-of-the-art of the

highly corrosive, it can be very efficiently used for the

S-technology is given by Lens & Hulshoff Pol (2000).

precipitation of heavy metals. Most bivalent heavy metal cations like, Ni, Pb, Cd, Cu, Zn, Co, exert extremely low solubility constants with S22. Therefore, S22 is an ideal ion

Nutrients

for precipitating heavy metals from process waters and ground waters. When present in a sufficiently high amount,

Water directives generally request a high degree of

the MeS can be recovered as raw ore for reuse in the metal

nutrients removal. Nutrients, however, are essential for

industry. An interesting full-scale example is the Zinc

agricultural production and represent a high economic

factory Budelco in Budel, The Netherlands, where both

value. Many regions of the world are facing a giant

the waste zinc and the sulphur is recovered to be reused as

imbalance in N and P and the closure of N and P cycles

raw ore and sulphuric acid in the metal industry (Figure 6).

will become a major challenge in the coming decades.

The developed S technology can also be used for

In this respect it should be noted that the high quality

selective recovery of specific metals from e.g. mining 2

activities. While traditionally OH is used for precipitating Meþ þ , MeS precipitation has some striking advantages over MeOH such as: † Extremely low solubility constants leading to very low residual Meþ þ concentrations, i.e., in mg/l instead of mg/l. † Recovery of metals as raw ore for reuse in the industry as smelters can use S precipitates.

P ores, which are now used for producing phosphates, are exhausted in 6 – 7 decades (Driver et al. 1999)! Fixing N for the production of artificial fertilisers is a very energy intensive production method. Remarkably, N from wastes and manure are often in excess at the same location where artificial fertilisers are used in huge amounts, e.g. in The Netherlands. This N imbalance requires a lot of fossil fuels for restoration, while short-cutting the N-loop is a much more obvious alternative. AD technologies are already playing a pronounced role in mineralising the organically

† Formation of compact MeS sludge with a very low sludge

32 bound N to NHþ is made free from the 4 , while PO4

volume index (SVI). MeOH sludges are generally very

biopolymers and polyphosphates. Digested manure, bio-

bulky and more difficult to dewater.

wastes, and sludges are valuable soil conditioners with a high nutritional value as very well understood by farmers for centuries. More recent is the recovery of nutrients from liquid steams, such as sewage, for fertilisation in irrigated agriculture (Van Lier & Huibers 2004, 2007). It should be noted that worldwide (peri)urban farmers largely use raw or (partially) treated municipal sewage for crop irrigation. In stead of at high energy costs destruction of the valuable nutrients, recovery is an efficient means towards regionally

Figure 6

|

Biological metal recovery applying the S conversion technologies in anaerobic-micro-aerobic biotechnology.

closing nutrients balances. In addition, it would bring economic benefits not only to the wastewater treatment

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system but also to the farmers. We recently estimated that Jordanian farmers could save e 650– 2000 per crop per


ACKNOWLEDGEMENTS

season when the nutrients in the partially treated sewage

The help of Yolanda Yspeert in preparing the anaerobic

are taken into account in their fertilisation scheme

reactor survey is highly acknowledged.

(Boom et al. 2007). In such set-up, the energy efficient anaerobic treatment technology could become a major player for the treatment of domestic sewage as nutrients

REFERENCES

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CONCLUSIONS † Anaerobic high-rate treatment for industrial wastewater can be considered a consolidated technology with sludge bed systems, like UASB and expanded bed reactors, as the commonly applied technology, reaching 90% of all installed reactor systems. † AD technology offers a potential cost-efficient first treatment step in closing process water cycles in a wide range of industries. † Anaerobic sewage treatment is a rapidly emerging technology, of particular interest for addressing the huge municipal flows in developing countries. A rapid increase in full-scale experiences consolidates the technology. † Energy conservation and the production of renewable energy carriers from waste streams and other biomass sources is nowadays a major incentive for applying AD technology. Whereas CH4 recovery for direct use and/or electricity generation via dual fuel motors or CHP is generally applied, novel research developments include H2 production and direct electricity. † Under reducing conditions heavy metals rapidly precipitate with sulphides leading to extremely low heavy metal concentrations in effluents. The technology creates possibilities for selective recovery of heavy metal resources from waste streams. 32 † Crop macro nutrients like NHþ 4 and PO4 are liberated

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