The wastewater recovery

After 10 dedicated years, Frank Rogalla of Aqualia hangs up his hat on Technically Speaking and looks to the future potential of energy and nutrient recovery in wastewater.

Silence is the biggest compliment you can expect from readers,” WWT’s then editor, Malcolm Hallsworth, told me ten years ago – when I started this monthly column. Almost 120 issues later, I am very grateful for the huge heap of compliments I got from WWT readers. However, apart from a handful of niggles when one product name was not sufficiently mentioned, I cannot recall any written responses to the Technically Speaking feature – so it is with immense gratitude for your patience and appreciation that I leave the scene for fresh ideas.

George Washington, the first President of the US, did not run for a third four-year term, stating that no man should have that honour for more than two terms. Thus, it became an “unwritten rule” in the US that no one should serve more than two terms as President – until the US constitution passed it into law the 22nd Amendment after Franklin D Roosevelt’s four term presidency 1933-1945.

And I must admit I did not manage to stay in one company more than nine years – at CGE (now Veolia) in France. Then I was sent to their subsidiary in the US that had just been acquired to help in technology transfer – so technically it was almost 12 years in the same group. At Black & Veatch (B&V), I could not resist the seven-year itch and left shortly before completing a full eight years.

Change is constant
The main reason for change is the feeling that one has contributed all that was possible in a given environment, and that hanging on would just produce more of the same. In the case of monthly articles, there is an expectation of a certain “newness” that invariably will be exhausted if relying on one single person for too long.

Some of you have probably noticed that my main ideas are frequently recycled, even though I am trying to enrich them with new perspectives and results – on nutrient recycle, membrane bioreactors, anaerobic digestion, energy balances and so forth.

During the years with B&V, it was reasonably easy to get original material as the company produces close to 50 articles a year for the annual conferences of the American Water Works Association and Water Environment Federation.

In addition, I have occasionally had help from other writers who let me use their ideas and materials – so a big thank you for that superb support. One of the main recurrent themes was nutrient removal and recovery, as I had the privilege of working with Dr James Barnard at Black & Veatch. Also, my first job at the research centre of CGE, 25 years ago, was to look at innovative ways to remove nitrogen and phosphorus from wastewater (see WWT June).

Notorious nutrients
This theme will remain an important one for the wastewater industry, not only because of the impact of nutrient discharges in the environment, but because of the value of these ingredients for food and energy production. When thinking of biofuel, most of the crops will have an energy content similar to wood, around 5kWh/kg. But biomass needs fertilizer, which in most cases is gained artificially through urea synthesis from atmospheric nitrogen, based on natural gas.

Estimates show that the production of ammonia consumes about 5% of global natural gas production. And of course world oil and gas reserves are estimated at just 45 years and 65 years respectively. The energy equivalent of ammonia is thus between 10 and 15kWh/kg N (depending on the efficiency of the urea plant), and the natural gas supply makes up about 90% of the cost of producing ammonia. In contrast, the biomass for biofuel production will contain a significant amount of nitrogen, varying between 5 and 10% for bacteria, and about half as much for wood or ten times less for grass.

If taking 5% nitrogen content in biomass as reference, the equivalent energy can be 15% of the net energy value – and considering that most energy conversions have an efficiency of 35%, the nitrogen input can be equivalent to almost half of the net energy potential of biomass.

While the phosphorus content in biomass is less significant, it is an even more limited nutrient, as it is entirely gained from mining, mainly in a dozen countries: in Africa – Morocco with 35% of world reserves, and South Africa, Senegal and Togo; in the Middle East -Egypt, Israel, Iraq and Jordan); and of course the big three – China, Russia and the US – mainly Florida. At the current rate of yearly consumption of phosphate rock of 40Mt/a phosphorus pentoxide (P2O5), the most optimistic experts predict that the world reserves will be depleted in about 350 years.

But if only the world’s annual diesel consumption, estimated around 700Bl/yr, were produced from biomass, containing about 1% of phosphorus, and with an oil content of 20% in the crop on a dry weight basis, the yearly need for phosphorus fertilizer for biodiesel would amount to 80Mt P2O5/year.

This would triple the current yearly phosphorus consumption. So even without any growth in food or biofuel consumption, the phosphorus reserves would be depleted in about 100 years. That is where recycled nutrients from humans or animals could come in handy. For six billion people, the potential production is 25Mt N/ yr , equivalent of 17% of artificial N fertilizer production.

The balance is even better for phosphorus, where the human potential of 4 to 5Mt P/ yr is equivalent to 25-30% of present yearly P production (the 40Mt /yr of phosphorus rock P2O5 would be 17t of P).

Once animal wastes are included, the potential nutrient recycle could be doubled. For instance, the two European countries with the biggest pig farms, Germany and Spain, is have about 26M animals each. Denmark boasts some 2.5 pigs per person.

And if you count your chickens – in 2002 there were nearly 16B chickens in the world, each one producing litter of around 100g/d – a total of 584M t/yr. If only 1% was phosphorus… And Professor Kroiss from the Technical University of Vienna estimates that if nitrogen recycling was targeted, urine separation has a maximum potential of substituting about 55% of market N-fertiliser – but at the cost of a significant change in infrastructure. Harvesting at source with new separation toilets could recover the 1 to 2l of daily liquid per person that contains 90% of the nitrogen, instead of flushing it to the wastewater plant, where it is released back to the atmosphere at significant energy expense (about 5kWh/kg N).

Nutrient recovery seems simpler for phosphorus, which is accumulated in sewage sludge. In Austria, about 85% of P in wastewater is contained in sludge, and using present technologies, could substitute around 40% of fertiliser imports (see WWT December 2010 ).

Adding to that the potential of phosphorus recycle from livestock, it should be possible to come close to self-sufficiency.

Eternal energy
The other value in wastewater is of course the organic energy, which in today’s systems is only partially harvested. Conventional wastewater treatment transforms dissolved organic matter into solids with considerable energy input, somewhere between 0.25 and 0.5 kWh/m3.

With luck, the transformation of the waste solids back to biogas by anaerobic digestion and co-generation covers about half of the electricity consumed at a WwTP. Although plant optimisation can even lead to self-sufficiency.

But the analysis of the potential energy content of raw wastewater (see Table 1) reveals that the 60g of solids that each of us dilutes in about 200/d of water should yield around 2.4kWh/m3 – five times more than the aeration energy. Similarly, if we would be able to transform all the chemical oxygen demand (COD) of raw wastewater into biogas, at a theoretical yield of 0.35m3 CH4/kg COD, the energy equivalent for 500mg COD/l would be 1.75kWh/m3.

Count to that the energy of the residual solids, which according to the table would have the same calorific content as wood.

Some countries have started to adopt their own pathway to wastewater treatment, rather than copying what was first implemented in England – for a specific climate with abundant rain and moderate temperatures. In Brazil, anaerobic pre-treatment is common, although the energy from biogas is rarely harvested, as machinery is considered expensive and difficult to operate and maintain, and fuel is plentiful and cheap. But this experience shows that 60-70% of the COD load, even for common concentrations of municipal wastewaters around 500mg COD/l, can be removed without energy input, yielding a very low production of solids: less than half of aerobic production.

This concept is of course easy to transfer to other geographies with similar characteristics, where wastewater temperatures rarely fall below 20°C, and anaerobic plants can be found in other South American countries, in Africa, Asia and the Middle East. Although research at the University of Galway in Ireland shows that anaerobic degradation of wastewaters can be equally efficient at psychrophilic temperatures below 15°C.

Two challenges remain, other than the usual problems of proper design, operation and maintenance, and the common syndromes of “not invented here” or “my wastewater is different than yours”:

  • To harvest the dissolved methane at lower temperatures, although stripping seems efficient if the air can then be used for combustion
  • For specific wastewaters with high sulphate content, a way to suppress the formation of hydrogen sulphide (H2S) has to be found, for instance by micro-aeration as practised in anaerobic sludge digestion

The other point is of course that an efficiency of two-thirds for the removal of pollution is not good enough, a polishing step is needed either for the removal of the remaining COD and solids, or for the recovery of nutrients and disinfection. Any conventional treatment can of course be added downstream, although new alternatives are being developed to achieve a better overall economic balance.

One elegant idea is to introduce a membrane into the anaerobic reactor, so that in one compact step a disinfected effluent rich in nutrients for reuse as irrigation water can be produced – leading potentially to an energy self-sufficient technology.

The other is to harvest the nutrients downstream, for instance in a photobioreactor to produce algae biomass – either for use in animal feed, chemical or pharmaceutical industries, or as biofuel.

Although the latter would have the lowest value, it is still better than producing sewage sludge, and is the theme of the large research project that I am leading now, financed by the 7th Framework Programme of the European Community. Hopefully I will be able to update you regularly, even if the main authorship of this column will be in better hands

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