Sludge and Biosolids Management

There is a need for proportionate and consistent approach to regulation

This is the second paper on this series. In the first1 I reviewed a broad breadth of issues related to managing sludge and biosolids including anaerobic digestion, the potential of biogas, drying and land application.

This paper will build on, rather than repeat the information in that first paper. Save to say that sludges are an inevitable consequence of treating wastewater and, because the type of wastewater treatment affects their character, consideration should be given to the fate of the sludge at the earliest stage in design of wastewater treatment. Having said that, there are exciting advances and retrofits that will be discussed in this paper.

Land application and sustainability

The phosphate industry has long realised that we are depleting the planet’s economic reserves at an unacceptably rapid rate 2 . Phosphorus is essential for all life because it is part of DNA; there is no substitute. Phosphate rock contains cadmium to a greater or lesser amount, so mining new rock brings ‘new’ cadmium into the anthropogenic cycle. Sewage sludge contains significant amounts of phosphate and, in fact, the most cost effective means of recycling phosphate and conserving it is to use biosolids (treated sewage sludge) on land as a phosphate fertiliser 3 . Modern discharge consents for many of the larger wastewater treatment works (WwTW) have increased the amount of P in sludge because they require that effluents only contain 1 or 2 mgP/l 4 .

The success of contaminant reduction as a result of controlling inputs at source and by hazardous substance legislation was discussed in the first paper. The fear of adventitious contaminants should no longer be a barrier to land application because they are excluded at source. If, as the phosphate industry says, the planet’s reserves of P are only 100 years at the present rate of exploitation, it must be verging on criminal irresponsibility to squander biosolids.

Biofuels have been in the news. The first generation biofuels use crops as feedstocks to produce fuels that are alternatives to those derived from fossil fuels. Brazil has used alcohol from sugar cane for many years because it has no indigenous fossil fuels. Diesel from oil seed rape (canola) is quite established in northern temperate areas. Production of alcohol from wheat, maize (corn) and sugar beet is increasing rapidly. This has had two effects, the world commodity prices of staple foods have increased and so have the world prices of mineral fertilisers.

As world populations grow by 50% (to 9 billion people) by 2050, weather for crop production becomes more uncertain and the area for rain-fed agriculture decreases, because of climate change, this tension is bound to increase. The second generation biofuels will be from cellulosic crop residues, etc., which will relieve the competition with food requirements but it could still deprive the soil of organic matter additions.

Biosolids and other organic resources now have higher real financial value to farmers and other land managers, in terms of fertiliser replacement, than for many years. Supplying farmers with biosolids involves a “technical sale” and the representative from the biosolids producer should be trained to give fertiliser advice.

Having said all of that, it is essential that producers treat and manage biosolids so that they are not unwelcome to people living near land application sites. Complying with regulations is a given, there are no prizes for not breaking the law, it is necessary to go beyond these minimum requirements to win trust and acceptance. The most important of these is odour. Odour is the factor most likely to draw public outrage. Odour is not inevitable and it is not regulated in most countries. Some treatment technologies produce biosolids with acceptable odour and others do not.

The Province of Quebec in Canada is perhaps the first to have a mechanism for regulating odour in land-applied organic resources including biosolids 5 . When solid biosolids are land applied there can be one hectare of treated surface, which is potentially odour generating, between the spreader and the ploughs, even with the best coordination. If local residents are outraged by odour they might turn to the web to find out more about sludge/biosolids and some of the sites they find will inflame their outrage. These sites might not be truthful but they are believable. This is a paradox, producers continue to select technologies that achieve the regulated requirements (bacterial counts, etc.) with little or no regard to the odour of the biosolids when they are land-applied, and at the same time they worry about the future for this market.

Independently audited Quality Management Systems and Quality Protocols are becoming means of “raising the bar” and differentiating good practice from less good. Examples are the Compost Quality Protocol in UK launched by WARP and the EA in 2007, the National Biosolids Partnership’s EMS for biosolids in the USA and ReVAQ (Clean Nutrients from Sewers) in Sweden 6 .

Boosting sludge digestion and biogas production

The first paper discussed sludge digestion and various innovations to increase the yield of biogas by destroying more of the feed solids. In particular this is related to increasing the breakdown of sludge from secondary treatment of wastewater. A technique that was not discussed is fitting “lysing knives” into the centrifuge thickening the secondary sludge. Prague WwTW and the Technical University pioneered this and it is now a commercial product that can be retrofitted into most makes of centrifuge (www.lysatec.com).

As the thickened surplus activated sludge (SAS) exits the thickening centrifuge the cells are smashed by the shear between the knives and the stator, which have a difference in speed of 2500 rpm. Retrofitting the lysing adaptation causes very little increase in the power consumption of the centrifuge. The effect of smashing the SAS is to reduce its viscosity so much that the digesters can be fed [with combined thickened primary and lysed SAS] at 10% dry solids and still be fully mixed, normally 6-7% would be the limit. It increases the treatment capacity of the existing asset by 40%.

Sludge treatment at Prague is by thermophilic anaerobic digestion (TAnD). 12 digesters operate as 6 in-series pairs. This reduces by-pass compared with operating the 12 in parallel. They are fed hourly. There is 55,000 m 3 digester capacity and they produce 50,000 N m 3 biogas/ day from which water is removed by condensers (6 °C water-cooled) and activated carbon removes siloxane (from 40 mgSi/N m 3 to 5). TAnD has proved stable. There is less foaming than there was with mesophilic anaerobic digestion (MAD). Volatile destruction (VS) is now about 60% and therefore more biogas; ‘conventional’ VS destruction is 40-45%. With TAnD there is less H 2 S but more siloxane (only 2ppm Si in MAD biogas) and also calcium carbonate which is thought to be aerosolised in the TAnD. This accumulates in the CHP engines, which require more frequent cleaning than they did with MAD.

The digested sludge is dewatered in centrifuges. The cake is 32%DS; it has low odour and is granular and not sticky. Normally (in the absence of SAS lysing) the cake from TAnD or MAD would be 23%DS, more odorous and sticky. Presumably, this is all a consequence of lysing the SAS. The biosolids are land applied. The combined effect of greater VS destruction and better dewatering mean that the quantity of cake produced is only 55% of that which would be produced with unlysed SAS.

The rise in energy prices and the need to divert food waste from landfill has started to increase acceptance that it is possible to get more out of existing biogas resources. Anaerobic digestion is the most common type of sludge treatment at the larger WwTW in Europe. They are normally fed at 6%DS but by retrofitting something like Cambi thermal hydrolysis (www.cambi.com) it is possible to at least double the %DS and also to comply with the Animal By-Products Regulation 7 .

The feed will have been sterilised at 160 °C for 20-30 minutes so the digestate will be pathogen free and it will dewater to around 34%DS. VS destruction will be at least 60%, i.e. at least 60% of the organic matter will be converted to biogas. Because of the enhanced VS destruction and the better dewatering (even though the mass of DS fed to the digesters has been doubled) the quantity of cake will be the same as when it was 6%, sludge-only feed, there will be about 3 times as much biogas and capital cost will be funded from gate-fees.

One crucial issue is removing plastic and other litter from the food waste; the best answer that I have seen to this challenging issue is Dewaster®, which was developed in Denmark and is now sold by Hese Umwelt (www. hese-umwelt.de). It is a high pressure screen press that extrudes the biodegradable pulp through a conical bar screen inside which there is a screw auger, the trash exits [dry] from the end of the press.

From a “helicopter perspective” this looks like a good solution for Society, it uses existing infrastructure to convert a waste into renewable energy and fertiliser, however with the silo-ism of policy and regulation there is a question about whether the biofertiliser is “sewage sludge” or “waste”. Earthworms do not know the difference, to them and the rest of the soil ecosystem it is just food, but policy makers have had difficulty looking at soil and crop protection from this perspective. Adoption of a Quality Protocol would answer that dilemma but that is for another article.

Cutting the intra-works cycling of N and P

Nitrogen (N) and phosphorus (P) in sludge dewatering liquor is at least 20% of the load of these elements on the WwTW when it is returned to the works’ inlet, which is the conventional routing for dewatering liquor. If the WwTW is a sludge treatment centre into which sludges are transported from outlying works, dewatering liquor represents and even greater proportion. The conventional route uses energy to treat this N and P by converting N to harmless nitrogen gas and capturing P into the biosolids. However, this nitrogen was “fixed”, it came from food and thus ultimately it was either fertiliser-N or N fixed by soil micro organisms; it is a pity to use energy to squander this fixed N if there is a cost effective alternative, and there is.

In 2006 I researched physico-chemical stripping of N and P from dewatering liquors for Mott MacDonald and Anglian Water 8 . VEAS WwTW in Oslo, Norway has practised ammonia stripping since 1998. VEAS has alkaline filtrate from its heated vacuum chamber presses because they lime the liquid digestate to sterilise it. Ammonia is stripped from this alkaline (pH 10) filtrate by spraying it down a tower against a current of air. The ammonia is then stripped from the air in a second tower with an acid spray. Originally, sulphuric acid was used but when farmers said they would prefer ammonium nitrate to ammonium sulphate, VEAS switched to nitric acid, which is supplied by Yara Fertiliser, which buys back the ammonium nitrate.

Ammonium nitrate has two drawbacks, a) the nitric acid embrittled the polypropylene towers and b) ammonium nitrate is an oxidiser and with organic matter will explode, so cleanliness is imperative.

Ammonium sulphate has become valued as a fertiliser once more because sulphur, which is an essential plant nutrient, is no longer supplied in rainfall becasue of clean air legislation. Ammonia stripping achieves greater than 90% N-removal at VEAS and is operationally very reliable.

Struvite (magnesium ammonium phosphate) can be a considerable nuisance in WwTW because it crystallises in pipelines carrying dewatering liquor if there is sufficient magnesium (there is always enough ammonium and phosphate) after any point of turbulence where CO 2 is exsolved. This can be turned from a problem to a benefit if the struvite is generated in a reactor. This can capture more than 90% of the P in the dewatering liquor and has been practised at operational scale in Japan since 1997. The struvite is sold as fertiliser 8 . Johnston and Richards (2003) 9 found that the P in struvite had the same fertiliser value as monocalcium phosphate fertiliser. Britton et al. (2007) 10 showed that struvite recovery has come of age in Canada and that the product commands a premium price as an amenity turf fertiliser.

Ammonia stripping and struvite recovery are well established operationally and they are ideally suited to be integrated into a combined liquor treatment plant. For optimal struvite recovery, the pH of the liquor has to be raised to about 9, partly by air-stripping CO 2 and partly by adding alkali. At pH 9 there will be approximately 40% ammonia (some of which will be in the off-gas) and 60% ammonium, by raising the pH to 10 after the struvite reactor, 95% will be ammonia and only 5% ammonium. Air is a carrier in all of this and the process efficiency can be increased by recirculating it so as to minimise the introduction of atmospheric CO 2 which minimises carbonate scaling and consumption of alkali.

The capital cost of a combined ammonium sulphate – struvite plant for 0.5 million population equivalent is expected to be not more than £1 million which is competitive with a biological plant. The revenue from selling products at only 50% of the farm gate prices would recover 75% of the cost of the input chemicals, maintenance and manning. The operating cost would be less than a biological plant and there is the bonus that the P content of the biosolids would be less than if the liquors were returned to the main wastewater treatment plant.

Summary

There is always something new in the management of sludge and biosolids. The value of the fertiliser content has increased considerably because of increased demand for food and energy crops and increased energy costs. There is also a sustainability imperative to recycle biosolids and fertiliser, not least because we are running out of phosphate rock.

The potential for generating biogas as renewable energy is under-realised and there is considerable potential to generate more though it is hindered by silo-thinking at policy and regulatory levels. Soil micro-organisms and plants do not recognise the origins of biofertilisers, they just ‘see’ plant nutrients and organic matter.

There is a need for proportionate and consistent approach to regulation and also for producers to ‘go the extra mile’ so that biofertilisers are not unwelcome when they are land-applied. Independently audited Quality Protocols will surely make a difference.

References

• Evans, T.D. (2006) Sludge and biosolids management, AWE International, March 2006, 15-19.
• Evans, T.D. and Johnston, A.E. (2004) Phosphorus and crop nutrition: principles and practices. In Phosphorus in Environmental Technologies: Principles and Applications. Valsami-Jones, E. (ed) IWA Publishing, ISBM 1 84339 001 9. London
• CEC (1991) Council Directive of 21 May 1991 concerning urban waste water treatment (91/271/EEC). Official Journal of the European Communities, No L135/40-52.
• Driver, J. Lijmbach, D. and Steen, I., (1999) Why recover phosphorus for recycling and how? Environ. Technol. 20 651-662
• Groeneveld, E. and Hébert, M. (2002) Classification of odours from residuals and compost compared to manure. Proc. Annual Conference of the Composting Council of Canada. Halifax, Nova-Scotia.
• Hugmark, P. (2006) ReVAQ – How EMS and Stakeholder Involvement is Getting Qualifying Biosolids Back onto Swedish Farms. Proc. 11th CIWEM Aqua-Enviro European Biosolids & Biowastes Conference. Wakefield, UK.
• CEC (2002) Regulation (EC) No 1774/2002 of the European Parliament and of the Council of 3 October 2002 laying down health rules concerning animal by products not intended for human consumption. Official Journal of the European Communities L 273/1 10.10.2002
• Evans, T.D. (2007) Recovering ammonium and struvite fertilisers from digested sludge dewatering liquors. Proc. IWA Specialist Conference: Moving Forward – Wastewater biosolids sustainability. June 2007. Moncton, NB, Canada
• Johnston, A.E. and Richards, I.R. (2003) Effectiveness of different precipitated phosphates as phosphorus sources for plants. Soil Use and Management, 19, 45-49
• Britton, A.T.; Sacluti, F.; Oldham, W.K.; Mohammed, A.; Mavinic, D.S. and Koch F.A. (2007) Value From Waste – Struvite Recovery at the City of Edmonton’s Gold Bar WWTP. Proc. IWA Specialist Conference: Moving Forward – Wastewater biosolids sustainability. June 2007. Moncton, NB, Canada

Published: 10th Sep 2007 in AWE International