Integrated Waste Management Volume II Part 8 potx

Anaerobic Processes for Waste Treatment and Energy Generation 239 The volumetric organic loading rate VOLR is the mass of dry organic feed/volume of digester/time, or n waste i oTS wast

Anaerobic Processes for Waste Treatment and Energy Generation 237

CO2 and CH4 bubbles that may attach to biomass and thus prevent settling The anaerobic contact process is a good choice for feeds with high suspended solids (e.g wood fiber), which enable microbes to attach to solids and settle Loading rates range from 0.5 to 10 kg COD/m3/day (Khanal, 2008)

Fig 7 Anaerobic contact process (after Khanal, 2008)

4.3.1.2.6 Anaerobic membrane bioreactor

An example of a suspended growth system, an anaerobic membrane bioreactor (AnMBR, shown in Figure 8a) uses a membrane, either within the reactor or in an external loop, to aid solids/liquid separation Since the membrane retains biomass, extremely long SRTs are possible regardless of the HRT (Khanal, 2008)

Fig 8 Schematics of (a) anaerobic membrane bioreactor, with membrane in an external loop, and (b) completely mixed bioreactor (after Khanal, 2008)

4.3.1.2.7 High-rate CSTRs

High-rate anaerobic digesters operated as completely mixed bioreactors, or completely stirred tank reactors (CSTRs), as shown in Figure 8b, have HRT=SRT They would thus be

suitable for high-solids feed streams (TS = 1-6%), including municipal sludge, animal manure, and other biowastes (Khanal, 2008) Required detention time is typically 15 days or less (Metcalf and Eddy, 2004) Mechanical mixing, pumping, and/or gas recirculation can provide mixing

4.3.2 Choose reactor material

The reactor must be airtight, since the methanogens are obligate anaerobes, and must also prevent liquids from leaking Considerations in choosing a material for the reactor include:

 Local availability

 Cost

 Ability to maintain heat (thermal insulation capacity)

 Ability to absorb solar radiation (light-colored materials can be painted black to increase solar energy absorption)

 Corrosion-resistant (hydrogen sulfide and organic acids associated with anaerobic degradation can cause corrosion)

Possible materials include:

 Brick and mortar (lime mortar with waterproofing can be substituted for cement where necessary)

 Concrete, sometimes with coating

 Glazed pottery rings cemented together

 Thick plastic (for very small tanks only)

Deublein and Steinhauser (2008) provide a more detailed discussion of reactor materials

4.3.3 Size reactor

The digester size can be estimated using a hydraulic retention time (HRT) or using volumetric organic loading rate (VOLR) Typically, both calculations are performed, and the larger of the two sizes is used, to be conservative

Typical HRTs for various wastes in anaerobic reactors were given in Table 2 Most of these HRTs are for the mesophilic temperature range Typical residence times for reactors operated in the mesophilic temperature range are from 20-45 days Typical residence times for reactors operated in the thermophilic range are around 15 days, since heating increases the rate of microbial activity From the HRT, the reactor volume VD can be estimated as (Deublein and Steinhauser, 2008):

VD = QTOTAL * HRT * 1.25 (30)

VD = M. TOTAL * HRT /water * 1.25 (31) where

QTOTAL = total waste stream (waste plus water) volumetric flow rate (m3/day)

1.25 is a factor to account for air and fixtures

MTOTAL = total waste stream (waste plus water) mass flow rate (mass/time)

Anaerobic Processes for Waste Treatment and Energy Generation 239 The volumetric organic loading rate (VOLR) is the mass of dry organic feed/volume of digester/time, or

n waste i oTS waste i TS waste i D

The average VOLR for small plants is 1.5 kg oDM/(m3*d) and for large plants is 5 kg oDM/(m3*d)

Once the digester volume is found, the dimensions of the digester can then be determined according to the following rule of thumb, assuming a cylindrical digester: HD = 0.5 * DD, where HD = height of the digester and DD = diameter of the digester

From Example 3, M. TOTAL = 3375 kg/day VD can then be calculated according to:

For this example,

VD = [M. septage * foTS septage * fTS septage + M. cow manure * foTS cow manure * fTS cow manure + .

Mpoultry manure * foTS poultry manure * fTS poultry manure + M. rice straw * foTS rice straw * fTS rice straw]/VOLR

VD = [225 *0.05 * 0.65 + 1187 * 0.135 + 9 * 0.45 * 0.75 + 432 * 0.375 * 0.825] kg/day /1.5

kgoDM/(m3*d) = 203 m3

To be conservative, the VD value of 211 m3 based on HRT will be used Assuming that the digester is cylindrical, VD = HD *  * DD2/4 Assume HD = 0.5 * DD Then,

VD = 0.5 * DD *  * DD2/4 = 0.125 *  * DD3

DD = [VD /(0.125 *  )]1/3 = [211 m3 /(0.125 *  )]1/3 = 8.13 m

HD = 4.06 m

4.3.4 Choose mixing method

In large reactors, mixing is useful in exposing new surfaces to bacterial activity and thus maintaining methane production rates Incorporating an agitator can considerably reduce the size of the reactor A rule of thumb is that if the volume exceeds 100 m3, mixer should be used (OLGPB, 1978) Mixing methods include:

1 Daily feeding of the digester (semicontinuous operation),

2 Installing a mixing device operated manually or mechanically,

3 Creating a flushing action of the slurry through a flush nozzle,

4 Creating mixing action by flushing the slurry tangentially to the digester content,

5 Installing wooden conical means that cut into the straw in the scum layer as the surface

of the liquid moves up and down during filling and emptying

Adequate mixing may be difficult to achieve in an undivided large digester (intended to serve an entire community, for example) Compartments may be particularly useful for large digesters producing >500 ft3 of gas/day

4.3.5 Determine heating requirements

Heating speeds the rate of methane production; thus, the detention time can be reduced and the digester size can be smaller than for an unheated unit However, heating takes energy The operational cost of providing this energy must be weighed against the reduced capital cost of a smaller digester For small digesters (producing <500 ft3 of gas per day), heating using fuel may not be desirable due to maintenance requirements Solar heating or use of waste heat from an engine-generator may be considered (NAS, 1977) Higher temperatures lower the amount of CO2 dissolved in the liquid phase, according to Henry’s law, and thus increases the percent in the gas phase; this lowers the energy content of the biogas per volume

The heat requirements for the digester include the amount needed (Metcalf and Eddy, 2004):

1 To raise the incoming slurry to desired digestion temperatures (qraise, or qR),

2 To compensate for heat losses through the reactor floor, walls, and roof (qlosses, or qL), and

3 To make up losses that might occur in piping between the heating source and tank (qpiping, or qP)

The total heat required is thus:

Heat required to raise the slurry temperature can be calculated from:

where qR = heat requirement, Btu/h (W)

MTOTAL = mass flow rate of slurry to be heated

Anaerobic Processes for Waste Treatment and Energy Generation 241

c = slurry heat capacity, which can be assumed to be the same as that of water (1 Btu/lb/F) (Metcalf and Eddy, 2004)

T = difference between the incoming slurry temperature and the desired reactor temperature

The maximum heat requirement should be calculated for the coldest month of the year Heat losses through the reactor floor, walls, and roof can be calculated according to:

qL = 1

n j

where qL = heat loss, Btu/h (W)

Uj = overall coefficient of heat transfer for surface j, Btu/ft2/h/F (W/m2/C)

Aj = cross-sectional area of surface j through which heat loss is occurring, ft2 (m2)

Tj = temperature drop across surface j, F (C)

Overall heat transfer coefficients for typical digester materials are given in Table 5 Expanded plastic slabs of polyurethane can provide insulation for the tank bottom For the upper portion

of the tank, expanded polystyrene slabs, mineral wool mats, plastic foam, leaves, sawdust, or straw can be used to insulate the tank and minimize heating requirements

Example 5

Continuing with the information from Examples 1-4, estimate the heat that would be required to heat the digester from 40F to 90F Assume that the digester is above ground, and made from 12” thick concrete walls with insulation The concrete floor is 12” thick, in contact with dry earth The fixed concrete cover is 4” thick and insulated Assume no losses between the heating source and tank

n j

UjAjTj

qL = UwallsAwallsTwalls + UfloorAfloorTfloor + UcoverAcoverTcover

From Table 5, taking the mean value in each range, Uwalls = 0.125, Ufloor = 0.06, and Ucover = 0.245 Btu/ft2/h/F

From Example 4, DD = 8.13 m and HD = 4.06 m The areas of the walls, floor, and cover are thus:

Awalls =  * DD * HD =  * 8.13 m * 4.06 m = 103.8 m2 = 1117 ft2

Afloor = Acover =  * DD 2/4 =  * (8.13 m) 2/4 = 51.9 m2 = 558.7 ft2

qL = 0.125 Btu/ft2/h/F * 1117 ft2 (90F – 40F) + 0.06 Btu/ft2/h/F * 558.7 ft2 (90F – 40F) +

0.245 Btu/ft2/h/F * 558.7 ft2 (90F – 40F) = 15,505 Btu/h = 646 Btu/day

qTOT = = 3.71 * 105 Btu/day + 646 Btu/day = 3.72 * 105 Btu/day

Plain concrete walls (above ground)

12” thick with air space plus brick facing 0.32-0.42

Plain concrete walls (below ground)

Plain concrete floors

12” thick, in contact with moist earth 0.10-0.12

Floating covers

With 1.5” wood deck, built-up roofing, and no insulation 0.32-0.35

With 1” insulating board installed under roofing 0.16-0.18

Fixed concrete covers

4” thick and covered with built-up roofing, not insulated 0.70-0.88

4” thick and covered, but insulated with 1” insulating board 0.21-0.28

Table 5 Overall heat transfer coefficients for typical digester materials(Metcalf and Eddy, 2004)

Anaerobic Processes for Waste Treatment and Energy Generation 243

4.4 Design the gas storage system

Gas can be stored in a digester with floating cover, or gas from a digester with a fixed cover can be piped into an auxiliary gas holder with a floating cover Materials for the cover can include mild steel, EDPM rubber, or concrete The volume of the gas holder depends on the daily gas production and usage It may be as low as 50% of the total volume of daily gas production, if gas usage is frequent

of 53.7 m3 For a cylindrical gas holder to fit onto the top of the digester whose dimensions were determined in Example 5, a suitable diameter would be 7.98m, or 15 cm less than the diameter of the digester The height of the gas holder would then be:

HH = VolH/( * DH2/4) = 53.7 m3/( * (7.98m)2/4) = 1.07 m

4.5 Determine system location

The system location should be:

 At least 50 ft from the nearest drinking water well, to avoid potential contamination (NAS, 1977)

 At least 10 m from any homes, to avoid any methane safety issues (FAO, 1984)

 Out of the sun in hot climates, in the sun in cooler climates (FAO, 1984)

 On firm soil, preferably with a low underground water level (OLGPB, 1978) Away from trees, so roots will not cause cracks (OLGPB, 1978)

 Close enough to place of use to reduce length of connection tubing, and corresponding loss in gas pressure associated with friction with the walls of the tube (OLGPB, 1978)

5 Benefits and limitations of anaerobic processes

Anaerobic treatment processes solve 2 problems at once: waste and energy Benefits of anaerobic processes compared to aerobic processes are discussed in detail in Sattler (2011), and are summarized briefly here Benefits of anaerobic systems compared to aerobic systems include:

 Production of usable energy,

 Reduced sludge (biomass) generation/stabilization of sludge,

 Higher volumetric organic loading rate/reduced space requirements,

 Reductions in air pollutants and greenhouse gases,

 Lower capital and operating costs,

 Lower nutrient requirements and potential for selective recovery of heavy metals Remaining limitations of anaerobic processes include:

 Requirements for post-treatment,

 Methane loss in the effluent,

 Sensitivity to low temperatures, and

 Attention required during start-up

6 Summary

Steps in anaerobic degradation of organic material by bacteria include polymer breakdown (hydrolysis), acid production (acidogenesis), acetic acid production (acetogenesis), and methane production (methanogenesis) Various factors associated with the waste impact both the quantity and rate of methane production, including waste composition/degradable organic content, particle size, and organic loading rate (kg/(m3*d) ) Environmental factors impacting the rate of methane generation include temperature, pH, moisture content, nutrient content, and concentration of toxic substances

Steps in design of a gas production system include:

1 Determine biogas production requirements,

2 Select waste materials and determine feed rates; size waste storage; determine rate of water addition and size the preparation tank,

3 Design the digester/reactor,

4 Design the gas storage system,

5 Determine system location

Benefits of anaerobic systems compared to aerobic systems include production of usable energy, reduced sludge (biomass) generation/stabilization of sludge, higher volumetric organic loading rate/reduced space requirements, reductions in air pollutants and greenhouse gases, and lower capital and operating costs

7 References

Barlaz, M A., Ham, R K., and Schaefer, D M (1990) Methane production from municipal

refuse: A review of enhancement techniques and microbial dynamics Critical

Reviews in Environmental Science and Technology, Vol 19, No 6, pp 557-584

Chan, G Y S., Chu, L M., and Wong, M H (2002) Effects of leachate recirculation on biogas

production from landfill co-disposal of municipal solid waste, sewage sludge and

marine sediment." Environmental Pollution, Vol 118, No 3, pp 393-399

Chugh, S., Clarke, W., Pullammanappallil, P., and Rudolph, V (1998) Effect of recirculated

leachate volume on MSW degradation Waste Management Research, Vol 16, No 6,

pp 564-573

Deublein, Dieter and Steinhauser, Angelika Biogas from Waste and Renewable Resources

Wiley-VCH, Weinheim, 2008

Dolfing, J “Acetogenesis.” In Biology of Anaerobic Microorganisms, edited by M.B Alexander

Zehnder, John Wiley & Sons, Inc., New York, U.S.A., pp 417-468, 1988

Faour, A A., Reinhart, D R., and You, H (2007) "First-order kinetic gas generation model

parameters for wet landfills." Waste Manage., 27(7), 946-953

Fernando, Sandun; Hall, Chris; and Saroj Jha “NOx Reduction from Biodiesel Fuels.” Energy

& Fuels, Vol 20, pp 376-382, 2006

Anaerobic Processes for Waste Treatment and Energy Generation 245 Filipkowska, U., and Agopsowicz, M H (2004) "Solids Waste Gas Recovery

Under Different Water Conditions." Polish Journal of Environmental Studies, 13(6),

663-669

Food and Agriculture Organization (FAO) of the United Nations Biogas, Vol 1 and 2, 1984

Gawande, N A., Reinhart, D R., Thomas, P A., McCreanor, P T., and Townsend, T G

(2003) "Municipal solid waste in situ moisture content measurement using an

electrical resistance sensor." Waste Manage., 23(7), 667-674

Gujer, W and Zehnder, A.J.B “Conversion processes in anaerobic digestion.” Water Science

and Technology, Vol 15, pp 127-267, 1983

Gurijala, K R., and Suflita, J M (1993) "Environmental factors influencing methanogenesis

from refuse in landfill samples." Environ Sci Technol., 27(6), 1176-1181

Henze, M.; Harremoes, P “Anaerobic treatment of wastewater in fixed film reactors – a

literature review.” Water Science and Technology, Vol 15, pp 1-101, 1983

Hulshoff Pol, L.W.; Lopes, C.I.S.; Lettinga, G.; Lens, L.N.P “Anaerobic sludge granulation.”

Water Research??, Vol 38, pp 1376-1389, 2004

Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report: Climate

Change 2007, http://www.ipcc.ch/publications_and_data/

publications_and_data_reports.shtml#1, accessed 2/11

Khanal, Samir Kumar Anaerobic Biotechnology for Bioenergy Production Wiley-Blackwell, 2008

Lettinga, G., Rebac, S., & Zeeman, G “Challenges of psychrophilic anaerobic wastewater

treatment,” Trends in Biotechnology, Vol 19 No 9, pp 363-370, 2001

McCarty, P.L and Smith, D.P “Anaerobic wastewater treatment: Fourth of a six-part series

on wastewater treatment processes.” Environmental Science and Technology, Vol 20,

No 12, pp 1200-1206, 1986

Mehta, R., Barlaz, M A., Yazdani, R., Augenstein, D., and Bryars, M (2002) "Refuse

Decomposition in the Presence and Absence of Leachate Recirculation." Journal of

Environmental Engineering, 128(3), 228-236

Metcalf & Eddy, Inc Wastewater Engineering: Treatment and Reuse Fourth Edition, revised

by George Tchobanoglous, Franklin L Burton, and H David Stensel McGraw Hill,

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Peoples’ Republic of China A Chinese Biogas Manual 1978

Sharma, K.R “Kinetics and Modeling in Anaerobic Processes” in Anaerobic Technology for

Bioenergy Production: Principles and Applications by S.K Khanal, Ames, Iowa: Blackwell, 2008

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J (2010) "Evaluation of Landfill Gas Decay Constant for Municipal Solid Waste

Landfills Operated as Bioreactors." Journal of the Air and Waste Management

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a Hot Climate, John Wiley & Sons, Chichester, England, 1994

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waste decomposition." Biosource Technological, (94), 69-81

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13

Management of Phosphorus Resources – Historical Perspective, Principal Problems and Sustainable Solutions

Yariv Cohen1,2, Holger Kirchmann1 and Patrik Enfält2

1Swedish University of Agricultural Sciences, Department of Soil and Environment

to sustain world food production (Heffer et al., 2006)

Mineable phosphate rock is a non-renewable resource However, a main proportion of the phosphorus fertiliser present in food ends up in urban wastes such as sewage sludge and slaughterhouse waste Urbanisation and population growth impose specific challenges for phosphorus recirculation At the global scale, more than 50% of the human population (3.3 billion) lives in urban locations and urbanisation is increasing (United Nations, 2010) In future, it will be of the utmost importance to recycle and reuse the phosphorus present in waste in order to minimise losses and conserve existing resources In fact, phosphorus recirculation in society already has a high priority in national environmental programmes However, the re-use of municipal wastes in agriculture is currently impeded by problems such as: (i) the presence of unwanted metals, organic pollutants and pathogens, limiting recycling of municipal wastes (sewage sludge, slaughterhouse wastes and household compost) to agricultural land; (ii) logistical difficulties in re-distributing surplus municipal wastes such as sewage sludge from urban areas back to arable land; and (iii) a low fertiliser value

Two contrasting situations for nutrient recirculation can be identified: huge urban centres with large-scale treatment of wastes requiring long-distance transportation of nutrients back

to arable land; and rural settlements with small-scale, on-site waste collection/treatment and sufficient arable land nearby for soil application This chapter mainly focuses on phosphorus recirculation from densely populated areas

The chapter begins by reviewing earlier waste treatment in society, the production of phosphorus fertilisers and foreseeable problems The conditions necessary to achieve recirculation of municipal wastes are then described and possible technical solutions that fulfil these conditions are presented

2 Historical perspective

2.1 Lesson from waste treatment in the past – limited recycling of human waste to soil

It could be assumed that in the pre-industrialised age, complete nutrient cycling was achieved through spreading human, animal and plant residues onto agricultural land However, recycling of human waste to land was limited in early societies

Urban settlements require wastes to be handled in a planned manner, which was the case even in early history The Indus and Harappa cultures, which settled along the Indus river (today Pakistan) around 3000 BC, seem to have used water to remove toilet wastes and conducted the wastewater into recipient water bodies (Glover & Ray, 1994) Houses with water toilets, bathrooms and outflows connected to brick-covered channels in streets have been found The Minoan culture on Crete in 2000-1500 BC also used water toilets, which were connected to sewage channels Stone-walled pits of about 5 m in diameter found

at Knossos were probably used for solid waste treatment through deep litter decomposition (Joyner, 1995) In the Greek and Roman cultures, town planning, water supply, sewage discharge and waste treatment were highly developed services Sewage water from Athens

in 500 BC was applied to open fields in rural surroundings (White-Hunt, 1980a), while drains and sewers of Nippur and Rome, among the great structures of antiquity, were used

to carry away storm runoff, toilet wastes and street washing water From the Cloaca Maxima in Rome (the main sewage tunnel), effluents were transported through channels

to far outside settlements for both discharge and infiltration (Dersin, 1997) Solid, settled waste material, ‘black gold’, was recovered from sewage systems and ponds and recycled

to arable land

Type of material

Mean water content (%)

Cadmium content (mg kg-1 dry

weight)

(mg kg-1phosphorus)

Harvested field crops:

Table 1 Some characteristics of municipal wastes compared with harvested field crops Data complied from: Kirchmann and Pettersson (1995), Kirchmann and Widen (1994), Cohen

(2009), Eriksson (2009) and Svanberg (1971)

In contrast, historical documents from China, Korea and Japan show comprehensive and effective handling and treatment systems for organic human wastes not using water for sewage transport (King, 1911) Instead, careful collection and extensive transport of latrine, organic wastes, ash, etc from some large cities back to agricultural land by human- or animal-drawn carts and manure boats is described Extensive collection was followed by careful storage and treatment Application of urine, pulverised human excreta, ash and

Management of Phosphorus Resources –

Historical Perspective, Principal Problems and Sustainable Solutions 249 composts, often mixed with sod or mud, canal sediments, etc., to arable land ensured a high degree of nutrient recirculation and maintenance of soil fertility It should be noted that the volume of waste transported back to agricultural land was larger than the volume of food consumed owing to the higher water content in different wastes compared with major food types (Table 1)

The Middle Ages were characterised by a decline in hygiene and sanitation standards in cities and towns in Europe Failure to remove the wastes from houses and streets, overloaded ditches and sewer channels in and around cities caused heavy pollution of watercourses in many places, for example London (White-Hunt, 1980a; 1980b) Wastes could be stored in tanks in the bottom of buildings or discharged into narrow lanes between houses from toilets placed above (the narrow alleys present in romantic medieval town structures) and the removal intervals could be long The absence of an effective sewage and waste handling system was a major hindrance in combating diseases in European cities of that era Furthermore, even animal wastes were not necessarily applied

to arable land, as a significant but unknown amount was leached to produce nitrate for use in gunpowder

In summary, early cultures discharged or infiltrated wastes and wastewater from based sewage systems outside urbanised areas and thus the nutrients they contained were not recycled to arable land Estimates show that at least 50% of total nutrients present in toilet wastes were lost, representing the proportion present in urine (see compilation by Kirchmann et al., 2005) The key lesson from this historical review is that recirculation of human wastes to soil was limited As a result, the stock of nutrients in agricultural soils was gradually depleted and soil fertility decreased

water-2.2 History of phosphorus fertiliser production - from bones to non-renewable

resources

To slow down nutrient depletion in arable soils, especially of phosphorus, animal bones

consisting of calcium phosphate were applied during earlier times Several 17th Century publications in Europe mention the beneficial effect of bones In 1769, the Swedish scientist J.G Gahn discovered that calcium phosphate is the main component of bones, but the role

of phosphorus as a major plant nutrient was still not known Field trials demonstrated that bones should be crushed and applied in the form of powder, but the positive effect obtained was ascribed to organic components in bones Attempts were made to improve the efficiency of bones by (i) composting them together with animal and plant wastes, (ii) boiling them in water; or (iii) treating them with steam under pressure The widespread use

of bones led to the idea of chemical treatment of bone material H.W Köhler of Bohemia was probably the first to suggest such a treatment and filed a patent for using acids (especially sulphuric acid) to process and produce commercial phosphate fertilisers (1831)

In 1840, Justus von Liebig published work showing that plants take up nutrients in the form

of inorganic components and carbon from air Until then, academics from Aristotle (384-322 BC) to Thaer (1752-1828 AD) had considered organic matter in soil (humus) to be the source of plant dry matter Liebig’s findings contributed to the acceptance and development of phosphorus fertilisers Together with the English businessman J Muspratt, Liebig developed and patented a method to produce a combined phosphorus and potassium fertiliser However, the fertiliser they produced was a complete failure, since the phosphate and potassium present were insoluble in water and therefore

unavailable to plants When the initial failure of this fertiliser and the insignificant effect

of bone powder as a fertiliser became understood, the importance of the water solubility

of plant nutrients was fully recognised and the concept of producing water-soluble

fertilisers was introduced (Finck, 1982)

Lack of bone material as a phosphorus source led to the import of guano from Peru around

1840 The discovery of low-grade mineral phosphates (apatite) in France and England eased the situation The first ‘artificial’ fertiliser, superphosphate, was produced in England in

1843 from apatite and sulphuric acid (see reaction below)

2 Ca5F(PO4)3 + 7 H2SO4  3 Ca(H2PO4)2 + 7 CaSO4 + 2 HF (superphosphate) (1) Superphosphate is a mixture of mono-calcium phosphate and gypsum, with a mean phosphorus content of 7-9.5% In 1855, superphosphate was also produced in Germany and

in 1860 the first plant was built in Sweden (Klippan) Due to increased use of artificial phosphorus fertilisers, cereal yields almost doubled between 1840 and 1880 from about 0.8

to 1.4 tons per hectare Use of phosphoric instead of sulphuric acid for apatite dissolution resulted in triple superphosphates being commercialised in 1890 These also consisted of mono-calcium phosphate, but without gypsum (see reaction below) and had a phosphorus content of 17-23%

Ca5F(PO4)3 + 7 H3PO4  5 Ca(H2PO4)2 + 2 HF (triple superphosphate) (2) The development of the phosphate industry was secured by the discovery of large sedimentary phosphate deposits in South Carolina (USA) Mining began in 1867 and by

1889 the USA was supplying 90% of the apatite used worldwide for phosphate fertiliser production

In 1917, a new phosphorus fertiliser was developed in the USA by reacting phosphoric acid with ammonia gas to form mono- and di-ammonium phosphate (see reactions below)

H3PO4 + NH3  NH4H2PO4 (mono-ammonium phosphate) (3)

H3PO4 + 2 NH3  (NH4)2HPO4 (di-ammonium phosphate) (4) Mono-ammonium phosphate is the inorganic phosphate salt with the highest phosphorus concentration (up to 26%) The production of ammonia on a major industrial scale from nitrogen gas in air and hydrogen gas in coal through the Haber-Bosch process boosted the production of ammonium phosphate fertilisers In 1926, IG Farbenindustrie in Germany announced the development of a series of multi-nutrient fertilisers based on crystalline ammonium phosphate In the late 1920s, the nitro-phosphate process was developed in Norway In this process, phosphate rock is treated with nitric acid and calcium nitrate and ammonium phosphate are produced (see reaction below)

Ca5F(PO4)3 + 10 HNO3  5 Ca(NO3)2 ↓ + 3 H3PO4 + HF (calcium nitrate) (5)

H3PO4 + NH3  NH4H2PO4 (mono-ammonium phosphate) (6) Reviews carried out by Finck (1982), Kongshaug (1985), and Mårald (1998) show that phosphate rock, a limited mineable resource, has been the main source for phosphorus fertiliser production since 1867

Management of Phosphorus Resources –

Historical Perspective, Principal Problems and Sustainable Solutions 251

3 Relevant issues

3.1 Rock phosphate and the cadmium and uranium problem

About 80% of the phosphate rock currently mined is used to manufacture mineral fertilisers Use for detergents, animal feeds and other applications (metal treatment, beverages, etc.) accounts for approx 12, 5 and 3 %, respectively (Heffer et al., 2006) The global production of rock phosphate amounted to 174 million tons in 2008 (IFA, 2010a) How long existing phosphorus reserves will last is difficult to forecast Some estimates vary between 50 to 100 years, assuming peak phosphorus (Cordell et al., 2009; Cordell, 2010) and excluding reserve bases currently not economical to mine (Steen, 1998; Driver et al., 1999; Stewart et al., 2005; Buckingham & Jasinski, 2006) Other estimates are around 350 years, based on current production capacity and excluding increased demand for phosphorus (IFDC, 2010; USGS, 2011)

Depending on its origin, phosphate rock can have widely differing mineralogical, textural and chemical characteristics Igneous deposits typically contain fluorapatites and hydroxyapatites, while sedimentary deposits typically consist of carbonate-fluorapatites collectively called francolite Sedimentary deposits account for about 80% of global production of phosphate rock (Stewart et al., 2005) As high-quality deposits have already been exploited, the quality of the remaining sedimentary phosphorus reserves is declining and the cost of extraction and processing is increasing, mainly due to lower phosphorus content in the ore (Driver et al., 1999) Associated heavy metals such cadmium and uranium substituting for calcium in the apatite molecule are often present at high levels in phosphate rock, especially that of sedimentary origin Rock phosphate may contain up to 640 mg cadmium per kilogram phosphorus (Alloway & Steinnes, 1999) and up to 1.3 g uranium per kilogram phosphorus (Guzman et al., 1995) Only a minor proportion of phosphorus reserves have low cadmium content (Fig 1) Most (85-90%) of the cadmium and uranium in rock phosphate ends up in fertilisers (Becker, 1989)

Brazil Russia South Africa Jordan Syria China Tunesia Israel Egypt USA Other countries Morocco and Western Sahara

Mineable phosphate rock as a proprotion

of total world reserves (%)

<10 mg Cd kg-1 P 10-50 mg Cd kg-1 P

>50 mg Cd kg-1 P

Fig 1.Mineable phosphate rock and cadmium content Estimates of mineable amounts taken from US Geological Survey (USGS, 2011) and cadmium contents from McLaughlin &

Singh (1996)

Recent studies show that uranium originating from fertilisers accumulates in soils, leading

to uranium losses to natural waters (Schnug & Haneklaus, 2008) The biochemical toxicity of uranium has been shown to be six orders of magnitude higher than the radiological toxicity (Schnug & Haneklaus, 2008) Uranium in soil enters the food chain mainly through consumption in drinking water

A new standard for low cadmium content in phosphorus fertilisers is likely to become an issue, since the European Food Safety Authority recently reduced the recommended tolerable weekly intake of cadmium from 7 to 2.5 micrograms per kilogram body weight, based on new data regarding the toxicity of cadmium to humans (EFSA, 2009) Several countries already restrict cadmium levels in phosphate fertilisers and there is a need for exclusion of cadmium and uranium from phosphorus fertilisers for safe food production

Fig 2 Urban and rural population of the world, 1950-2050 Data from United Nations (2010)

3.2 Population growth and urbanisation

The global population is rapidly increasing Between 1950 and 2009 the population increased from 2.5 billion to 6.8 billion and it is expected to reach 9.1 billion by 2050 (United Nations, 2009) In addition, the 20th Century witnessed rapid urbanisation in the world The proportion of urban population increased from 13% in 1900 to 29% in 1950 and reached 50%

in 2009 (United Nations, 2010) Population growth is expected to occur mainly in urban areas (Fig 2), the population of which is projected to increase from 3.4 billion in 2009 to 6.3 billion in 2050 Cities in less developed regions will become centres of population growth Table 2 shows the expected population growth for some large cities between 2010 and 2025 Statistics show that 1.4 billion people live in 600 cities, excluding suburban areas with a population larger than 0.75 million inhabitants (mean population of 2.3 million per city) (GeoHive, 2010)

Urbanisation and population growth impose specific challenges for phosphorus use: (i) long-distance recycling of nutrients from large cities back to arable land to avoid

contamination of surrounding areas and to ensure long-term supply of P fertiliser; (ii)

increase in crop production by at least 50% by 2030 to ensure sufficient food supply