Phosphate Recovery
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Phosphates recovery for recyling from sewage and animal wastes
Alone
amongst organisms modern man has forgotten the importance of conserving
and re-using phosphorus. Estimates of the Earths phosphate reserves vary
considerably but most commentators expect them to last little more than
one hundred years. Moreover, it is certainly the case that the highest
quality reserves are being depleted rapidly and the way we currently use
phosphate does not accord with the principles of sustainable development.
The opportunity exists to reverse this trend via the development of effective
sewage and animal waste treatment technologies which facilitate the removal
of phosphorus from these waste steams into a form suitable for recycling
by the phosphate industry.
John Driver, Manager, Business
Planning and Environment, Albright & Wilson, Warley, UK, looks at the
issues and current developments in this field, which were the subject of
the first
International Conference on the Recovery of Phosphates
for Recycling from Sewage and Animal Wastes,
organised by the Centre Européen dEtudes des Polyphosphates
(CEEP), held at Warwick University in the UK on 6-7 May, 1998.
Why should anyone bother about phosphorus recovery?
Historically, phosphate has been inexpensive and seemingly inexhaustible. Its dramatic effect on soil fertility and hence agricultural production has ensured that its use has been liberal even profligate. Phosphorus addition to soils in Europe has, for many years, far exceeded agronomic requirements with the agricultural community regarding phosphate fertilizer addition as an insurance policy: it costs very little, will not do any harm (to the crop) and may just do some good.
With such a large amount of phosphate
in the Earths crust, why should this be a concern? Even making no allowance
for global population increase, phosphate reserves are being rapidly depleted.
Opinions vary concerning the lifetime of known exploitable deposits yet
at present extraction rates (around 40 million t/a P2O5) todays reserves
are unlikely to last much more than one
hundred years.
Perhaps, like petroleum, our view of what is, or is not, commercially exploitable will change with time but what is patently clear to anyone who has worked in the phosphate industry for a number of years is that the quality of commercial phosphate rock is declining inexorably.
This decline reveals itself in a number of ways: the P2O5 content of phosphate rock is reducing and the highest grade ores are attracting a premium; beneficiation techniques are being applied to a higher proportion of recovered rocks; the levels of certain impurities, which pose problems in processing (e.g. magnesium), or in application (e.g. cadmium and uranium) are increasing. At the same time, there is growing pressure on the presence of some of these impurities in finished products and on the disposal of impurities extracted during processing or purification (phosphogypsum, fluoride and heavy metals).
For the non-fertilizer sector of
the phosphate industry, techniques for the extraction of impurities
from green phosphoric acid are
at an advanced state of development.
But, inevitably, for every Cinderella stream of purified phosphoric acid
there is an ugly sister metal-enriched stream. The disposal route for
these metals is usually into fertilizers but, with increasing pressure
on the cadmium content, this route may not be available indefintely. For
those companies without a fertilizer sink there has been a need to develop
alternative technologies to extract and isolate problematic metals in an
environmentally safe form: but at a cost. Even without the additional burden
of impurities from the purified acid sector, metal levels in fertilizers
are under attack.
There are, therefore, three fundamental reasons why the phosphate industry should regard the status quo as unsustainable:
l Growth in the world population, particularly in areas with poor soils, will lead to an increase in phosphate fertilizer consumption, only partially offset by a declining rate of use in the historically over-fertilized temperate zones.
l The pressure to remove heavy metals from all phosphate products (including fertilizers) derived from natural phosphate rock will lead to increasing raw material prices and escalating disposal costs.
Over three decades there has been
persistent pressure on the non-fertilizer sector of the phosphate industry
concerning the contribution of detergents to phosphorus levels in rivers
and lakes. Increasingly this is also being felt by the fertilizer sector.
Even farmers, traditionally a no-go area
for environmental legislation, are
beginning to feel the heat with respect to balanced fertilization, slurry
spreading etc. European water legislation concerning nutrients is diffuse
and, perhaps with the exception of the Urban Waste Water Directive
(91/271/EEC) and the Nitrates Directive (which is principally concerned
with risks
to health), has gone largely unnoticed.
The proposed Water Framework Directive, currently at an advanced
draft state, is intended to gather together much of this disparate legislation
into a coherent whole. Looking forward, we should expect to see phosphorus
control, probably phosphorus removal, becoming a requirement at all significant
sewage treatment works discharging into inland waters. Stricter controls
on intensive livestock farming may also become the norm and, as localised
manure surpluses pose increasing challenges for disposal, treatment systems
and phosphorus removal are likely to feature in future thinking.
Where phosphorus removal is required by law, phosphorus recovery from sewage treatment systems may be an economically attractive alternative. Animal wastes also offer a potentially large source of phosphates for recovery. The technology of phosphate recovery is relatively straightforward and, as well as the value of the recovered phosphorus, there can be significant savings in both treatment costs and in the disposal of the residual sewage sludge.
Just how much phosphorus is out there?
The quantities of phosphorus present
in sewage and animal waste are significant compared with the needs of the
non-fertilizer sector
of the phosphate industry. In the
UK around 40 million tonnes of domestic sewage (i.e. organic waste prior
to dilution with tap or rain water) is produced each year
as well as some 150 million tonnes
of farm animal wastes in livestock
units. These wastes can be estimated to contain around 45,000 and 200,000
tonnes, respectively, of phosphorus (as P). Together this represents approximately
six times the consumption of phosphate products (excluding fertilizers)
in the UK. The pattern in other European countries is likely to be broadly
similar although the balance between the human and the animal contributions
may vary and, on a local
or regional basis, may be markedly
different.
Realistically, not all of this phosphorus would be available for recovery, Even if practical techniques could be developed, logistic factors, such as the cost of transport and the scale of installations, would make recovery inappropriate. In practice, recycling is likely to be an economic option only in the case of large, geographically concentrated waste streams (sewage from urban areas, intensive livestock units). In rural areas, agricultural sludge or manure spreading will probably always remain the best option for recycling nutrients.
However, in the UK even a conservative estimate of the potential for phosphorus recovery and recycling (50% recovery applicable to 25% of sewage and to 15% of animal wastes) represents half of industrial phosphate demand. Putting this into perspective, fertilizers and animal feeds consume at least five times as much phosphate as industrial applications.
What phosphorus recycling strategies are there?
Spreading of sewage sludge and animal manures onto agricultural land has always been, and will remain, the simplest strategy for recycling nutrients and the importance of this route has been recognised throughout history. Compelling archaeological evidence exists to show that rigid procedures existed for the gathering and re-application of manure (both human and animal) to farmed land. In English monasteries, by the Middle Ages, it was commonplace for contracts, governing the use of monastic lands, to contain a manure clause which permitted the farmer to graze his sheep, take away the wool, take away the meat, but the manure had to stay where it fell: on the field.
Things have changed, however. The growth of cities has resulted in the centres of consumption (and hence, human sewage production) becoming remote from areas of agricultural production This has led to logistical difficulties in restoring human waste to the land a problem, incidentally, made more difficult with the widespread introduction of sewage treatment. In recent years, the whole practice of sludge spreading has been called into question with pressure on heavy metal content, pathogens, odour, nutrient losses to water etc.
Manure and slurry spreading might seem a comparatively homely and unproblematic practice. But here, too, problems have arisen due to the intensification of livestock production particularly pigs and poultry. This has resulted in large local excesses of manure production, far beyond the capacity of nearby farmland to absorb the output. In such situations, alternative disposal routes have to be found; incineration is amongst the options already being employed.
The excess of nutrients, currently
from sewage treatment works but increasingly from animal wastes, will need
to be prevented from reaching surface water if quality objectives are to
be achieved and maintained. Hitherto, the strategy has been one of phosphorus
removal, not recovery and this has been achieved via a variety of methods
In all cases, however, where phosphorus is removed from waste waters it
is transferred to sludge, either in an organic form, as in biological phosphorus
removal, or as a chemical precipitate: usually in
the form of an iron or aluminium
salt. The majority of sewage
works equipped with phosphorus removal
in Europe use chemical precipitation, often simultaneous with secondary
biological treatment, where the chemical precipitate is mixed into the
organic sludge. Effective phosphorus removal requires higher concentrations
of precipitation chemicals than actually combine with the available phosphorus
and, without exception, these methods result in a large (around 40%) increase
in sludge production. The resulting sludge is of dubious agronomic value
and presents its own disposal problems.
Recovery of phosphorus for recycling, rather than its transfer into sewage sludges, may offer economic and environmental rewards for the water industry. For the phosphate industry it holds out the promise of a significant, if only partial, source of sustainable raw material, which is comparatively free from heavy metals. These benefits must be compared with the investment and running costs of phosphorus recovery installations.
Routes to phosphorus recovery
Phosphorus removal by traditional precipitative methods will generally preclude phosphorus recovery for recycling by the phosphate industry. The resulting iron or aluminium compounds are incompatible with technologies currently used in the phosphate industry. They either require excessive energy input, to separate the phosphates from the added precipitation chemicals, or interfere with the industrial process. As a rule of thumb, for either the solvent extraction or electrothermal reduction routes to high purity phosphate compounds, a maximum of 1-2% of iron and aluminium would be considered the upper limit. Incinerated sewage sludge, generated through a precipitation route might easily contain 20% iron (as Fe203).
A surprisingly large amount of work
has been carried out on possible routes to phosphorus recovery. Many of
these have never been developed beyond the pilot, or even laboratory, scale
but several
have been implemented at both demonstration
and full scale. In general, these processes isolate the recovered phosphorus
in the form either of a calcium phosphate or magnesium ammonium phosphate
(struvite).
As a preliminary step to generating a concentrated liquid phosphate steam for phosphate recovery, biological phosphate removal techniques look very promising. In a side steam, the nutrient rich sludge can be made to yield a liquor containing phosphorus in excess of 100 mg/l at least a tenfold concentration increase compared with raw sewage, and all achieved isothermally! Such a stream would be particularly appropriate for phosphorus recovery.
Struvite formation as a route to phosphorus recovery
Full-scale struvite recovery processes are already operational or being built in Japan and the Netherlands:
l The Unitika Ltd (Osaka) struvite precipitation process is already in application at the Ube Industries Sakai plant (industrial waste waters) and is due to be commissioned in September 1998 at the Shimane Prefecture sewage works, Japan (45,000 m3/d).
l The Geochem Research/Delft University Earth Sciences stirred precipitation process produces potassium struvite from 700,000 t/a of calf manure at Putten in the Netherlands (early 1998).
Doubts remain, however, about the
value of recovered struvite for the phosphate industry. Whilst struvite
undoubtedly has some value as a fertilizer (including its ammonia and magnesium
content), it is difficult
to imagine how it might be transformed
into other phosphate derivatives using any existing technology at the disposal
of the industry. There is no reason whatever to believe that struvite could
be used in a traditional wet acid route. Whilst it is conceivable that
struvite might be capable of processing through the electrothermal reduction
route, the presence of ammonia in the molecule would suggest some interesting
challenges in furnace feed preparation (e.g. NOX scrubbing) or a radically
different process flowsheet.
Nevertheless, struvite remains an intriguing opportunity, if for no better reason than that it forms itself spontaneously in sewage treatment works. It has been described as an accident waiting to happen and may, in theory, be harvestable. The same cannot be said of the other route: calcium phosphate formation.
Calcium phosphate formation as a route to phosphorus recovery
Several pilot and full scale processes have been tested or are already operating in different countries, recovering phosphates from waste water streams through calcium phosphate formation. Samples of recovered materials from several units have been examined. All were pellet-like solids which drain readily to below 5-10% water and can offer 5-15% phosphorus content.
Fluid bed crystallisation installations
l Pilot plant developed by DHV and Essex and Suffolk Water (Compagnie Générale des Eaux) at Chelmsford sewage works, UK, 1997-1998.
l Experimental pilot reactor developed and tested for CEEP by Karlsruhe University at Darmstadt Süd sewage works, Germany, 1997-1998.
l Demonstration plant developed at Warriewood, Australia, by Sydney Water (50,000 pe, 1995- 1996).
l Three plants constructed by Kurita, Japan.
From the phosphate industrys point of view, a recovered calcium phosphate is an ideal form for onward processing as it is indistinguishable, in most respects, from mineral phosphate rock. In some examples of recovered calcium phosphate, residual organics levels have been comparatively high and, untreated, this would pose problems for the wet acid route. A simple calcining step as employed on many natural rocks may need to be employed. For the electrothermal reduction route a high residual organics level should pose no problems.
The economic case for phosphorus recovery
The economics of the recovery of phosphorus are not determined, in the main, by the value of the recovered phosphate. On the whole, the economic boundary conditions are defined by the cost of sludge disposal (incineration is more costly than landfill or agricultural spreading) where phosphorus recovery will normally result in reduced sludge production, and the competing costs of phosphorus precipitation chemicals (iron or aluminium salts versus lime). Given a choice, the water industry will generally adopt the cheapest available option which can consistently meet regulatory requirements. A great deal of further work needs to be done in the development of a robust economic model but the following conclusions can be drawn:
l Phosphorus recovery will only be viable where phosphorus removal is mandatory; it is never likely to compete with alternative sewage treatment methods where a lower level of treatment is permitted.
l Application of the EC Directive on the Treatment of Urban Waste Waters (91/271) will significantly increase the number of sewage works where recovery might be considered in the years ahead.
l Sewage sludge disposal options and costs, including investment and running costs for thickeners and digesters will be the decisive variable in the economic equation.
l The scale of installations and collection costs for recovered phosphates mean that phosphorus recovery is most likely to be attractive in medium to large sewage works.
l Phosphorus removal will be more easily integrated into biological P-removal systems or in sewage works where phosphorus removal has not yet been installed.
l Phosphorus recovery can facilitate nitrogen removal, via struvite, and is more easily integrated into plants fitted with nitrogen removal.
l Phosphorus recovery may, under certain circumstances, offer the water industry a removal and disposal route for heavy metal contaminants.
l Higher waste water phosphorus contents will render recovery more attractive.
l The phosphate industry will have to evolve and modify its structure in order to establish a stable market for recovered phosphate products.
l Although there is no conceivable health risk from industrial recycling of phosphates from sewage (no pathogen can possibly survive the pH and temperature conditions of the chemical processes used by the phosphate industry), this does not eliminate the possibility of adverse public reaction to the concept of recovering ingredients from sewage and animal wastes for uses such as detergents, food additives, animal food supplements. This will have to be addressed.
The use of recovered phosphates, produced in appropriate chemical forms, should not present a processing problem for the phosphate industry. It would, however, necessitate a major redefinition of the industrys structure and logistics. The European phosphate industrys structure has increasingly moved towards a small number of large production plants. There is only one remaining phosphorus furnace site in Western Europe (Vlissingen, Holland) and less than a dozen phosphoric acid plants (phosphoric acid is more commonly imported from production plants situated alongside the phosphate rock mines). In the long term, the use of small, diffusely produced quantities of recovered phosphates would necessitate restructuring the phosphate industry. Small local production units and appropriate stocking, grouping and transport systems may be needed.
In the short term, to get recovery started, it may be more appropriate for recovered products to be recycled locally, via small scale manufacture, into relatively simple fertilizers for agricultural or horticultural use.
Where do we go next?
As well as the further work on the economic aspects of recovery, a number of research priorities have been identified including: a better understanding of chemical and physical reaction conditions which promote crystallisation of phosphates; a clearer picture of the segregation of heavy metals in sewage treatment regimes; the role of organic materials in precipitation or crystallisation processes; the role of different seed materials (sand, calcite) in promoting recovery; a better knowledge of the solubility phases of different calcium phosphates; the agricultural value of struvite and the need to study its dissolution behaviour (nutrient release) in soils and accessibility for crops; a better understanding of the conditions for nucleation and growth of struvite under real sewage treatment works conditions.
More important than all of these, however, is the need to create a will to make phosphorus recovery happen.
The non-fertilizer phosphate industry is convinced that the future lies in phosphate recycling and considers that within a decade up to 25% of phosphates used in detergents and other high-grade applications could be recovered from sewage and animal wastes. But the industry cannot make this happen on its own. A partnership is required between the industry and the water treatment sector to begin to recover and, just as important, begin to create a market for recovered phosphate materials. Above all, government involvement: both national and European, is needed to provide the leadership and the policy stimulus, to make phosphorus recovery a reality. n
Phosphate recovery installations
(from top) at Geestermerambacht municipal waste water treatment plant.
Edam. Holland; Warriewood sewage works, near Sydney, Australia; and Shimane
Prefecture sewage works. Osaka, Japan. (bottom): Recovered phosphate pellets
(magnified).
John Driver
Albright & Wilson UK
Ltd.
P.O. Box 3, 210-222
Hagley Road West
Oldbury, Warley
West Midlands, B68 0NN
john_e_driver@albriw.com