Biological Denitrification of Potable Water Using Coconut Shells in a ...
Biological Denitrification of Potable Water Using Coconut Shells in a ...
Biological Denitrification of Potable Water Using Coconut Shells in a ...
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BIOLOGICAL DENITRIFICATION OF POTABLE<br />
WATER USING COCONUT SHELLS IN A<br />
FLUIDISED BED<br />
Abstract<br />
Nitrate concentrations <strong>in</strong> surface water and especially <strong>in</strong> ground water are a rapidly grow<strong>in</strong>g<br />
environmental problem. Concerns <strong>of</strong> possible health consequences have led to the adoption <strong>of</strong> a<br />
str<strong>in</strong>gent nitrate standard (50 mg nitrate (NO - 3 ) per litre) <strong>in</strong> water for human consumption (WHO,<br />
1984). <strong>Biological</strong> denitrification processes <strong>of</strong>fer a potential alternative to the relatively costly and<br />
<strong>in</strong> some cases <strong>in</strong>effective physicochemical treatment processes. <strong>Biological</strong> denitrification utilises<br />
bacteria to denitrify the aqueous nitrates <strong>in</strong> the absence <strong>of</strong> oxygen. This research will exam<strong>in</strong>e the<br />
use <strong>of</strong> coconut shells as the carbon source for denitrify<strong>in</strong>g bacteria, as well as provid<strong>in</strong>g a<br />
medium on which the bacteria can grow.<br />
The focus <strong>of</strong> this research is on the physical and environmental factors that <strong>in</strong>fluence the<br />
effectiveness <strong>of</strong> biological denitrification. This research is attempt<strong>in</strong>g to characterise these<br />
factors, and <strong>in</strong> do<strong>in</strong>g so, evaluate and optimise the performance <strong>of</strong> the fluidised bed reactor. It is<br />
envisaged that design guidel<strong>in</strong>es for determ<strong>in</strong><strong>in</strong>g the most effective temperature, hydraulic<br />
residence time, and nitrate load<strong>in</strong>g to operate the fluidised bed bioreactor, will be developed.<br />
The scientist who is content merely to carry out his work, without stopp<strong>in</strong>g to ask himself, `To what<br />
effect, for good or ill among men, is my work be<strong>in</strong>g applied?’ is no more than a ditch digger with a<br />
diploma <strong>in</strong> his pocket. Frank Dalby Davison, `Literature’, Soviet Culture: Selection <strong>of</strong> Talks at the<br />
Cultural Conference, 1942
Introduction<br />
Nitrate (NO - 3 ) is a naturally occurr<strong>in</strong>g form <strong>of</strong> nitrogen found <strong>in</strong> the soil. Nitrogen is essential to<br />
all life, and most crop plants require large quantities to susta<strong>in</strong> high yields. The formation <strong>of</strong><br />
nitrates is an <strong>in</strong>tegral part <strong>of</strong> the nitrogen cycle <strong>in</strong> our environment. A moderate amount <strong>of</strong> nitrate<br />
is a harmless constituent <strong>of</strong> food and water. Plants use the nitrates from the soil to satisfy nutrient<br />
requirements, and may accumulate nitrate <strong>in</strong> their leaves and stems. The nitrate is released <strong>in</strong> a<br />
soluble form when the plant degrades.<br />
Due to its high solubility, nitrate can also leach <strong>in</strong>to groundwater. Nitrate pollution <strong>of</strong> dr<strong>in</strong>k<strong>in</strong>g<br />
water constitutes an important and rapidly grow<strong>in</strong>g environmental problem. As a result <strong>of</strong> human<br />
activities, the nitrate contam<strong>in</strong>ation <strong>of</strong> groundwater is becom<strong>in</strong>g more prevalent. In agricultural<br />
regions the extensive and uncontrolled use <strong>of</strong> fertilisers <strong>of</strong>ten leads to nitrate leach<strong>in</strong>g <strong>in</strong>to the<br />
groundwater. Similarly the discharge <strong>of</strong> ammonia-rich effluents <strong>in</strong>to the soil is another<br />
significant source <strong>of</strong> nitrate pollution, typically from poorly designed or ma<strong>in</strong>ta<strong>in</strong>ed municipal<br />
sewage treatment networks.<br />
High concentrations <strong>of</strong> nitrate <strong>in</strong> water can cause methemoglob<strong>in</strong>emia or blue baby syndrome, a<br />
condition especially found <strong>in</strong> <strong>in</strong>fants under six months. The stomach acid <strong>of</strong> an <strong>in</strong>fant is not as<br />
strong as older children and adults, which causes an <strong>in</strong>crease <strong>in</strong> bacteria that can readily convert<br />
nitrate to nitrite (NO - 2 ). The nitrite is absorbed <strong>in</strong>to the blood, and haemoglob<strong>in</strong> (the oxygencarry<strong>in</strong>g<br />
component <strong>of</strong> blood) is converted to methemoglob<strong>in</strong> which is unable to carry oxygen<br />
(White et al. 1973). This results <strong>in</strong> reduced oxygen supply to vital tissues, such as the bra<strong>in</strong>.<br />
Methemoglob<strong>in</strong> <strong>in</strong> <strong>in</strong>fant blood cannot be changed back to haemoglob<strong>in</strong>, a process that normally<br />
occurs <strong>in</strong> adults. Severe methemoglob<strong>in</strong>emia may result <strong>in</strong> bra<strong>in</strong> damage and death by oxygen<br />
starvation.<br />
Pregnant women, adults with reduced stomach acidity, and <strong>in</strong>dividuals deficient <strong>in</strong> the enzyme<br />
that changes methemoglob<strong>in</strong> back to normal hemoglob<strong>in</strong> are all susceptible to nitrate-<strong>in</strong>duced<br />
methemoglob<strong>in</strong>emia. The obvious symptom <strong>of</strong> methemoglob<strong>in</strong>emia is a bluish colour <strong>of</strong> the<br />
sk<strong>in</strong>, particularly around the eyes and mouth. Other symptoms <strong>in</strong>clude headache, dizz<strong>in</strong>ess,<br />
weakness and difficulty <strong>in</strong> breath<strong>in</strong>g.<br />
Bottle-fed <strong>in</strong>fants are predisposed to sudden <strong>in</strong>fant (cot death) death syndrome, a condition <strong>of</strong><br />
oxygen starvation, from which about 3 per 1000 <strong>in</strong>fants from most European societies die<br />
(Money 1978). Many explanations for the occurrence <strong>of</strong> the syndrome have been <strong>of</strong>fered, but<br />
none appear satisfactory. Because <strong>of</strong> the similarity <strong>in</strong> symptoms it is possible that high<br />
methaemoglob<strong>in</strong> levels may predispose <strong>in</strong>fants to sudden death syndrome (WHO 1978).<br />
2
Healthy, well-nourished adults can consume fairly large amounts <strong>of</strong> nitrate with few known<br />
health effects. In fact, most <strong>of</strong> the nitrate we consume <strong>in</strong> our diet is from raw or cooked<br />
vegetables. The nitrate is readily absorbed and excreted <strong>in</strong> the ur<strong>in</strong>e. However prolonged <strong>in</strong>take<br />
<strong>of</strong> high nitrate levels has been l<strong>in</strong>ked to gastric problems due to the formation <strong>of</strong> nitrosam<strong>in</strong>es.<br />
These N-nitrosam<strong>in</strong>e compounds have been shown to cause cancer <strong>in</strong> test animals (Self and<br />
Waskon, 1992).<br />
Concerns over possible health consequences has led to the adoption <strong>of</strong> a str<strong>in</strong>gent nitrate standard<br />
(50 mg nitrate (NO - 3 ) per litre) 1 <strong>in</strong> water for human consumption (WHO, 1984). In the United<br />
States, the Environmental Protection Authority (USA EPA) has adopted an allowable nitrate<br />
concentration <strong>of</strong> 10 mg/L <strong>of</strong> NO - 3 -N (EPA, 1986). In New Zealand the M<strong>in</strong>istry <strong>of</strong> Health has a<br />
Maximum Acceptable Value for nitrate <strong>in</strong> dr<strong>in</strong>k<strong>in</strong>g water <strong>of</strong> 50 mg/L nitrate (M<strong>in</strong>istry <strong>of</strong> Health).<br />
In New Zealand, Rosen (1997) assessed 72 national sites from the National Groundwater<br />
Monitor<strong>in</strong>g Network (NGWP), and found that 6% <strong>of</strong> these sites had concentrations greater than<br />
11.3 mg/L <strong>of</strong> NO - 3 -N. Similarly Pa<strong>in</strong>ter et al. (1997) found that the follow<strong>in</strong>g regions typically<br />
had nitrate concentrations greater than 10 mg/L <strong>of</strong> NO - 3 -N; the Amuri Pla<strong>in</strong>s, Ashburton, Ashley<br />
catchment, Hamilton Bas<strong>in</strong>, Heretaunga Pla<strong>in</strong>s, L<strong>in</strong>coln, Pukekohe, Takapau Pla<strong>in</strong>s, and the<br />
Waimea Pla<strong>in</strong>s.<br />
The removal <strong>of</strong> nitrates is an expensive process and for this reason, nitrate standards are <strong>of</strong>ten not<br />
enforced, even <strong>in</strong> countries where the standard has been <strong>of</strong>ficially adopted. Inexpensive and easy<br />
to apply nitrate technologies are therefore urgently required. <strong>Biological</strong> processes <strong>of</strong>fer a<br />
potential alternative to the relatively costly and <strong>in</strong> some cases <strong>in</strong>effective physicochemical<br />
treatment processes that currently exist.<br />
1 Equivalent to 14mg/L NO 3 - -N<br />
3
Background<br />
<strong>Water</strong> is an essential element <strong>in</strong> the ma<strong>in</strong>tenance <strong>of</strong> all forms <strong>of</strong> life. To susta<strong>in</strong> life the available<br />
water must have specific characteristics. <strong>Water</strong> quality is def<strong>in</strong>ed <strong>in</strong> terms <strong>of</strong> those<br />
characteristics.<br />
S<strong>in</strong>ce World War II and the dawn <strong>of</strong> the 'chemical age', water quality has been heavily impacted<br />
world-wide by <strong>in</strong>dustrial and agricultural chemicals. Eutrophication <strong>of</strong> surface waters from<br />
human and agricultural wastes, and nitrification <strong>of</strong> groundwater from agricultural practices has<br />
affected large parts <strong>of</strong> the world.<br />
Literature reviews (Bouwer and Crowe, 1988, Gayle et al., 1989, Kapoor and Viraraghavan,<br />
1997) have identified that removal <strong>of</strong> nitrates from dr<strong>in</strong>k<strong>in</strong>g water is an important and develop<strong>in</strong>g<br />
area <strong>of</strong> research. They identified a need to further optimise current treatment technologies as well<br />
as to develop new and emerg<strong>in</strong>g technologies.<br />
<strong>Biological</strong> denitrification processes <strong>of</strong>fer a potential alternative to the relatively costly and <strong>in</strong><br />
some cases <strong>in</strong>effective physicochemical treatment processes. Nitrate is a stable and highly<br />
soluble ion, with low potential for coprecipation or absorption. These properties make it difficult<br />
to remove us<strong>in</strong>g conventional water treatment technologies such as lime s<strong>of</strong>ten<strong>in</strong>g and filtration.<br />
More sophisticated technologies such as – chemical denitrification, ion exchange, reverse<br />
osmosis, electrodialysis, catalytic denitrification – can be used to remove nitrates from dr<strong>in</strong>k<strong>in</strong>g<br />
water. However these methods have several disadvantages rang<strong>in</strong>g from excessive operational<br />
costs, problem associated with the waste disposal <strong>of</strong> by-products, and operational limitations.<br />
One <strong>in</strong>novative and emerg<strong>in</strong>g technology is biological denitrification, Bouwer and Crowe (1988)<br />
identified that detailed research is required to evaluate nitrate removal rates, with respect to<br />
substrate concentrations (a rate limit<strong>in</strong>g nutrient to growth), pH, and temperature. The <strong>in</strong>fluence<br />
<strong>of</strong> toxic organic compounds on reactor performance is also still unknown. In addition, they<br />
identified concerns with the deterioration <strong>of</strong> the microbiological characteristics <strong>of</strong> biologically<br />
denitrified water.<br />
This research differs from current research <strong>in</strong> this field because the coconut shell has a dual role,<br />
both as the substratum (the support surface on which the microbial cells grow), as well as the<br />
substrate, s<strong>in</strong>ce the coconut shell’s provid<strong>in</strong>g the organic carbon source for the bacteria. Exist<strong>in</strong>g<br />
research has considered supplement<strong>in</strong>g the biological process with an external carbon source<br />
such as methanol, ethanol, or acetic acid. Thus all the current biological denitrification design<br />
relationships are unable to be applied to this research.<br />
4
<strong>Biological</strong> <strong>Denitrification</strong><br />
<strong>Biological</strong> denitrification utilises bacteria to denitrify aqueous nitrates <strong>in</strong> the absence <strong>of</strong> oxygen.<br />
The bacteria are heterotrophic (more correctly chemoheterotrophic), that require an energy<br />
source, either <strong>in</strong> the form <strong>of</strong> organic carbon, carbon dioxide or sulphur. These denitrify<strong>in</strong>g<br />
bacteria are a form <strong>of</strong> anaerobic respirators, that use nitrate and/or nitrite as electron acceptors for<br />
the oxidation <strong>of</strong> organic compounds when oxygen is absent, produc<strong>in</strong>g nitrogen gases (N 2 , NO,<br />
and N 2 O). The microbial reduction <strong>of</strong> nitrate to gaseous nitrogen products is termed dissimilatory<br />
denitrification or nitrate respiration. Generally denitrification is considered to be an anoxic<br />
process, occurr<strong>in</strong>g <strong>in</strong> the presence <strong>of</strong> nitrate and the absence <strong>of</strong> molecular oxygen. The process<br />
proceeds through a series <strong>of</strong> four steps, from nitrate to nitrogen gases.<br />
Nitrate NO<br />
−<br />
3<br />
→ NO<br />
−<br />
2<br />
→ NO → N<br />
2O<br />
→ N<br />
2<br />
Nitrogen Gas<br />
Surpris<strong>in</strong>gly, most <strong>of</strong> the organisms known to denitrify are not strictly anaerobes, but rather are a<br />
form <strong>of</strong> facultative organisms, which under anoxic conditions use nitrate as a f<strong>in</strong>al electron<br />
acceptor for anaerobic respiration (Green et al., 1994). Gayle et al. (1989) discovered that most<br />
<strong>in</strong>vestigators consider oxygen as an <strong>in</strong>hibitor <strong>of</strong> denitrification, some species have been reported<br />
to denitrify <strong>in</strong> systems with oxygen tensions as high as 0.2 bar 2 .<br />
S<strong>in</strong>ce denitrify<strong>in</strong>g bacteria are heterotrophs, they utilise organic carbon to produce new biomass.<br />
These organisms break down complex organic molecules <strong>in</strong> a series <strong>of</strong> steps. They ga<strong>in</strong> most <strong>of</strong><br />
their energy from respiration by pass<strong>in</strong>g electrons along a series <strong>of</strong> electron acceptors (electron<br />
transfer cha<strong>in</strong>) to oxygen or <strong>in</strong> this case nitrate, which is reduced to nitrogen gases and water. In<br />
this way chemical energy is converted to adenos<strong>in</strong>e triphosphate (ATP) 3 . The yield <strong>of</strong> ATP per<br />
unit <strong>of</strong> idealised organic substrate (glucose) is less when nitrate is the term<strong>in</strong>al acceptor, than if<br />
oxygen is available, however anaerobic respiration still rema<strong>in</strong>s a very efficient energy yield<strong>in</strong>g<br />
process.<br />
As mentioned, the organic carbon is chemically oxidised, while the nitrate ion is reduced to<br />
ultimately gaseous nitrogen and water. Therefore the form <strong>of</strong> the organic carbon, can affect the<br />
rate <strong>of</strong> denitrification (Æsøy et al. 1998).<br />
Most <strong>of</strong> the experience with biological denitrification <strong>in</strong> dr<strong>in</strong>k<strong>in</strong>g water has been <strong>in</strong> Europe.<br />
These denitrification units are primarily anoxic biological granular media filters and fluidised-<br />
2 1bar is equivalent to 0.9869 atm, where 1 atm is the pressure <strong>of</strong> 1m <strong>of</strong> water<br />
3 ATP, is the <strong>in</strong>tra-cellular chemical unit used to exchange energy between cellular functions<br />
5
ed reactors. As <strong>in</strong> most potable water sources, the water is low <strong>in</strong> organic carbon and requires<br />
the addition <strong>of</strong> an organic carbon source for denitrification. Methanol has frequently been<br />
applied, however ethanol and acetic acid have also proved to be effective due to their<br />
comparative low cost and reduced toxicity (Bosman and Hendricks, 1981).<br />
While the operat<strong>in</strong>g costs <strong>of</strong> a biological denitrification reactor is substantial cheaper then other<br />
physicochemical alternatives, the majority <strong>of</strong> the operat<strong>in</strong>g cost is associated with the<br />
consumption <strong>of</strong> methanol. Therefore a bioreactor to denitrify contam<strong>in</strong>ated dr<strong>in</strong>k<strong>in</strong>g water, that<br />
uses a cheaper, non-toxic carbon source <strong>of</strong>fers significant advantages over the current options.<br />
Carbon Sources<br />
Researchers (Volokita et al., 1996a, 1996b) <strong>in</strong> Israel have <strong>in</strong>vestigated the denitrification <strong>of</strong><br />
dr<strong>in</strong>k<strong>in</strong>g water us<strong>in</strong>g newspapers and cotton as the energy source. Their experiments <strong>in</strong>volved<br />
pack<strong>in</strong>g laboratory-scale columns with shredded newspapers or unprocessed short fibre cotton.<br />
The media <strong>in</strong> the columns provided the sole chemical and physical substrate for the microbes.<br />
These series <strong>of</strong> experiments exam<strong>in</strong>ed the use <strong>of</strong> the simple cellulose component <strong>of</strong> these<br />
mediums as the source <strong>of</strong> organic substrate needed by the heterotrophic bacteria. S<strong>in</strong>ce cellulose<br />
is a basic component <strong>of</strong> all plant materials, it is an abundant and renewable resource.<br />
Cellulose is a l<strong>in</strong>ear glucose polymer with hydrogen bond<strong>in</strong>g between hydroxyl groups <strong>of</strong><br />
neighbour<strong>in</strong>g parallel cha<strong>in</strong>s. Lign<strong>in</strong> 4 and hemicellulose occur <strong>in</strong> close association with the<br />
cellulose fibres. The arrangement <strong>of</strong> these three organic molecules <strong>in</strong>creases the ability <strong>of</strong> the<br />
cellulose to resist enzymatic attack, thus only specific bacterial groups hav<strong>in</strong>g the required<br />
enzymes have the capacity to degrade cellulose.<br />
This research makes use <strong>of</strong> a readily available, but an unutilised resource found <strong>in</strong> the most<br />
‘develop<strong>in</strong>g’ or ‘third world’ countries <strong>in</strong> the Asia-Pacific region. <strong>Coconut</strong> shells are an organic<br />
woody product, with similar properties to wood. In wood, cellulose constitutes about 50 percent<br />
<strong>of</strong> the wood by weight. Lign<strong>in</strong> makes up about 23 to 33 percent <strong>of</strong> s<strong>of</strong>twoods but only about 16<br />
to 25 percent <strong>of</strong> hardwoods. Lign<strong>in</strong> is an <strong>in</strong>soluble, <strong>in</strong>tractable material that is difficult to remove<br />
from the wood as it b<strong>in</strong>ds the wood fibers together.<br />
Hemicelluloses are associated with cellulose and make up about 15 to 30 percent <strong>of</strong> the wood by<br />
weight. Ash-form<strong>in</strong>g m<strong>in</strong>erals are found <strong>in</strong> wood <strong>in</strong> very small quantities – perhaps 0.1 to 3<br />
4 Lign<strong>in</strong> is the material that stiffens the cell-walls <strong>of</strong> woody tissue <strong>in</strong> plants<br />
6
percent by weight. Extractives are not part <strong>of</strong> the wood structure, but they contribute colour and<br />
odour to the wood (Infopedia, 1997).<br />
Reactor Configurations<br />
Traditionally biological denitrification processes utilise fixed bed biological filters. However<br />
fixed bed reactors tend to suffer from clogg<strong>in</strong>g – where the growth <strong>of</strong> bacteria is sufficiently<br />
large to block the pore spaces between the packed medium. This results <strong>in</strong> <strong>in</strong>creased hydraulic<br />
shear stresses <strong>in</strong> the adjacent pore pathways. If the shear stresses are sufficiently large the<br />
bacteria attached to the support media is sheared <strong>of</strong>f, thereby decreas<strong>in</strong>g the efficiency <strong>of</strong> the<br />
reactor. As <strong>in</strong>dicated <strong>in</strong> Table 1, the current practice is to rigorously backwash the filter medium<br />
<strong>in</strong> fixed bed reactors, to remove a proportion <strong>of</strong> the immobilised biomass.<br />
Fluidised bed reactors <strong>of</strong>fer superior performance compared to complete-mix and fixed-bed<br />
bi<strong>of</strong>ilm reactors because the bi<strong>of</strong>ilm is evenly distributed throughout the reactor; while the liquid<br />
regime <strong>of</strong> a fluidised bed <strong>of</strong>fers all the advantages <strong>of</strong> a plug-flow reactor.<br />
S<strong>in</strong>ce the media is <strong>in</strong> a turbulent fluidised state, clogg<strong>in</strong>g does not occur. Instead the hydraulic<br />
conditions are kept symmetrical <strong>in</strong> the reactor – where the support particles order themselves<br />
vertically up the reactor (i.e. the larger more dense particles settle near the bottom and the lighter<br />
less dense particles rise to the top). Because fluidised beds do not suffer from clogg<strong>in</strong>g, it is to be<br />
expected that they would be capable <strong>of</strong> greater denitrification rates.<br />
Comparison <strong>of</strong> Different Biomass Support Reactors<br />
Fixed/Expanded Beds<br />
Fluidised Beds<br />
Biomass control Back-wash<strong>in</strong>g Cont<strong>in</strong>uous particle removal and clean<strong>in</strong>g<br />
Biomass recovery Wash<strong>in</strong>gs collected <strong>in</strong> humus tank At high concentration external to reactor<br />
Bed biomass attached<br />
to substratum<br />
Variable<br />
Steady (variable if there is periodic<br />
particle removal)<br />
Reactor performance<br />
(e.g.: carbon removal)<br />
Variable<br />
Steady (variable if there is periodic<br />
particle removal)<br />
Design<br />
Requires prediction <strong>of</strong> the variation <strong>of</strong><br />
the bed biomass hold-up with time<br />
Requires prediction <strong>of</strong> the variation <strong>of</strong><br />
the bed-particle biomass hold-up with<br />
time<br />
Table 1 Comparison <strong>of</strong> the different support media reactors. (Source: Cooper P.F. and B. Atk<strong>in</strong>son, 1981)<br />
In fact, Bouwer and Crowe (1988) observed that fluidised bed reactors appear to <strong>of</strong>fer the highest<br />
nitrate-load<strong>in</strong>g rate per unit volume, while overcom<strong>in</strong>g the potential for clogg<strong>in</strong>g <strong>of</strong>ten<br />
7
experienced <strong>in</strong> static bed reactors. Fluidised bed reactors however require greater recirculation <strong>of</strong><br />
the effluents to achieve sufficient nitrate removal, thus they require more process control.<br />
Traditional biomass reactors, like fixed or expanded beds, tend to operate with variable biomass<br />
concentrations, s<strong>in</strong>ce periodic back wash<strong>in</strong>g <strong>of</strong> the biomass <strong>in</strong>to humus collection tanks<br />
decreases the mass <strong>of</strong> biomass. As Table 1 <strong>in</strong>dicates, fluidised beds operate with a constant<br />
concentration <strong>of</strong> biomass <strong>in</strong> the reactors.<br />
8
Scope and Statement <strong>of</strong> Objectives<br />
Both Bouwer and Crowe (1988) and Kapoor and Viraraghavan (1997), have reviewed current<br />
nitrate removal processes and concluded that biological denitrification <strong>of</strong>fers a viable alternative<br />
to current technologies. They identified however several research needs for biological<br />
denitrification; these <strong>in</strong>clude the evaluation and optimisation <strong>of</strong> the performance <strong>of</strong> fluidised bed<br />
and other reactors, and the determ<strong>in</strong>ation <strong>of</strong> the k<strong>in</strong>ds <strong>of</strong> natural and synthetic organic<br />
compounds that can be biotransformed.<br />
The scope <strong>of</strong> this research is dist<strong>in</strong>ctly different from exist<strong>in</strong>g research <strong>in</strong> this field. The coconut<br />
shell has a dual purpose, not only is it a substrate (a rate-limit<strong>in</strong>g nutrient for growth), it is also<br />
the substratum (the support surface on which the microbial cells grow). Oppos<strong>in</strong>g gradients <strong>of</strong><br />
nitrate and organic carbon nutrients will occur with<strong>in</strong> the bacterial layer. Predict<strong>in</strong>g the rate <strong>of</strong><br />
bacterial growth and similarly the rate <strong>of</strong> denitrification are therefore complicated by the<br />
availability <strong>of</strong> both the nitrate and organic carbon substrates.<br />
Bouwer and Crowe (1988) identified fluidised bed reactors as <strong>of</strong>fer<strong>in</strong>g the best load<strong>in</strong>g rate per<br />
unit volume. An up-flow fluidised bed reactor will be constructed, because it overcomes the<br />
clogg<strong>in</strong>g problem experienced by static-beds. The reactor will utilise the denitrification<br />
properties <strong>of</strong> anaerobic respir<strong>in</strong>g bacteria consum<strong>in</strong>g the pelletised shell <strong>of</strong> coconuts <strong>in</strong> an anoxic<br />
environment.<br />
The specific objectives <strong>of</strong> this research <strong>in</strong>clude:<br />
1. The construction and operation <strong>of</strong> an anoxic fluidised bed reactor that would treat <strong>in</strong>fluent<br />
concentrations <strong>in</strong> the range <strong>of</strong> 20-200mg/l Nitrate (NO - 3 ).<br />
2. Characteris<strong>in</strong>g the performance <strong>of</strong> the fluidised bed <strong>in</strong> terms <strong>of</strong> the load<strong>in</strong>g rates (nitrate and<br />
carbon consumption), denitrification rates, and hydraulic residence times.<br />
3. Determ<strong>in</strong><strong>in</strong>g the effluent quality <strong>in</strong> terms <strong>of</strong> the chemical oxygen demand, nitrate<br />
concentration, suspended solids concentration, PO 3- 4 concentration, and dissolved oxygen.<br />
4. Classify<strong>in</strong>g the degradation <strong>of</strong> the coconut shell.<br />
5. Assess<strong>in</strong>g the resilience <strong>of</strong> the reactor to shock load<strong>in</strong>g, either by vary<strong>in</strong>g the hydraulic<br />
retention time, temperature, or the concentrations <strong>of</strong> the <strong>in</strong>fluent nutrients.<br />
6. Investigat<strong>in</strong>g the <strong>in</strong>fluence <strong>of</strong> utilis<strong>in</strong>g the support particles as a carbon substrate source - the<br />
advantage/disadvantage <strong>of</strong> this development and the effect <strong>of</strong> diffusion limitations to<br />
produce local variations <strong>in</strong> the microbial species with<strong>in</strong> the biomass.<br />
7. Modell<strong>in</strong>g the denitrification process, <strong>in</strong> particular the <strong>in</strong>fluence <strong>of</strong> the rate <strong>of</strong> bacterial<br />
growth with the assimilation <strong>of</strong> nitrate and organic carbon nutrients.<br />
9
Description <strong>of</strong> the Reactor<br />
A cont<strong>in</strong>uous fluidised bed reactor will keep the coconut shell fragments <strong>in</strong> suspension. As<br />
mentioned, the advantage <strong>of</strong> fluidised beds over packed reactors is that the electron acceptor<br />
(nitrate enriched fluid) and the support medium (and substrate) are evenly distributed <strong>in</strong> the<br />
reactor, thus allow<strong>in</strong>g the biological denitrification process to proceed with greater efficiency.<br />
The reactor is moulded from Pyrex® borosilicate glass and modular <strong>in</strong> construction. Borosilicate<br />
glass was selected due to the ease <strong>of</strong> construction, as well as its high resistance to attack from<br />
water, acids, salt solutions, and organic solvents. Each <strong>of</strong> the modular sections is flanged to allow<br />
the modules to be easily connected and <strong>in</strong>terchanged. A photo <strong>of</strong> the reactor configuration is<br />
shown <strong>in</strong> Figure 1. Latex gaskets seal across the adjacent glass flanges – with a sta<strong>in</strong>less steel<br />
clamp provid<strong>in</strong>g the positive force to hold the flanges together.<br />
A synthetic <strong>in</strong>fluent feeds the reactor at a rate <strong>of</strong> 0.033 to 23.5 L/day depend<strong>in</strong>g on the required<br />
hydraulic residence time. A stock solution <strong>of</strong> concentrated nitrate and phosphorous that can be<br />
added periodically and diluted <strong>in</strong> a known volume <strong>of</strong> deionized tap water, and stored <strong>in</strong> an 80L<br />
<strong>in</strong>fluent storage conta<strong>in</strong>er. A Masterflex® pump then pumps the <strong>in</strong>fluent via an <strong>in</strong>l<strong>in</strong>e pipe mixer<br />
<strong>in</strong>to the reactor. The <strong>in</strong>l<strong>in</strong>e pipe mixer will ensure that the recycle l<strong>in</strong>e and <strong>in</strong>fluent l<strong>in</strong>e are<br />
sufficiently mixed so that a homogeneous solution flows to the reactor.<br />
The homogeneous solution then enters the reactor through a diffusion chamber. A series <strong>of</strong> baffle<br />
diffusers <strong>in</strong> the diffusion chamber provide uniform velocity <strong>in</strong> the tube <strong>of</strong> the reactor. The<br />
Pyrex® glass reactor tubes be<strong>in</strong>g constructed from two flanged glass segments, allows various<br />
comb<strong>in</strong>ations <strong>of</strong> reactor lengths to be erected.<br />
The upflow<strong>in</strong>g fluid holds the coconut shell pellets <strong>in</strong> a fluidised state with<strong>in</strong> the reactor tube.<br />
The reactor has been designed so that all the biological denitrification occurs with<strong>in</strong> the tube <strong>of</strong><br />
the reactor. If however any <strong>of</strong> the media (coconut shell pellets) escapes the reactor tube, the<br />
settl<strong>in</strong>g chamber will reta<strong>in</strong> the media and return it back to the reactor tube. The <strong>in</strong>crease <strong>in</strong><br />
cross-sectional area <strong>of</strong> the bowl-shaped settl<strong>in</strong>g reactor allows the velocity <strong>of</strong> the upflow fluid to<br />
reduce sufficiently, so that the coconut shell pellets fall out <strong>of</strong> a fluidised state and settle back<br />
<strong>in</strong>to the reactor tube.<br />
10
Figure 1 Photograph <strong>of</strong> the fluidised bed reactor and pump configuration.<br />
Gauze filters on the <strong>in</strong>let <strong>of</strong> the effluent and recycle l<strong>in</strong>es will reta<strong>in</strong> the majority <strong>of</strong> the biomass<br />
and coconut shell fragments with<strong>in</strong> the reactor prevent<strong>in</strong>g the biological process from occurr<strong>in</strong>g<br />
outside the reactor. The recycle flow will be ma<strong>in</strong>ta<strong>in</strong>ed by pass<strong>in</strong>g the l<strong>in</strong>e through a recycle<br />
pump, and then back <strong>in</strong>to the reactor. The effluent will run <strong>in</strong>to a collected conta<strong>in</strong>er for later<br />
analyses.<br />
11
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13