Developement of processes for nutrient removal from wastewater

M.S.Le　M.L.Yu　H+Zheng

【Abstract】This review focuses on techniques for achieving very low concentration of TN and TP in water.The 1970 Clean Water Act in the US and the Urban Waste Water Treatment Directive in Europe now provide the benchmark standards for TN and TP levels.Arden and Lockett discovered activated sludge in the 1910s.Since then，many improvements have been made to the biological treatment process to provide various configurations that have the ability to deliver effluent concentrations between 6-8mg TN/L and 0.5-1.0mg TP/L，without external carbon addition.Typically， however， additional advanced nutrient removal technologies are used in tertiary treatment to meet more stringent effluent quality requirements.One advanced technology that has been successfully implemented in the US is denitrifying filters.For TP removal，tests showed that the BluePRO and CoMag systems both reliably met an effluent target as low as 0.04 mg TP/L.However，the establishment of stringent TN and TP effluent limits will dramatically increase the capital and operational costs of wastewater treatment plants.A more promising nutrient capturing process is based on the assimilation of nutrients （both N and P）during heterotrophic growth which uses a carbon source for energy.Laboratory trials of the single step process by Reach Green suggest that concentrations as low as 0.5 mg TN/L and 0.02mg TP/L are readily achievable.

【Key words】Nutrient removal；Wastewater；Environment；Phosphorus；Treatment standards；Denitrification；Carbon source

1 Introduction

The water environment is affected by every activity that takes place on land as well as through our actions in abstracting， using and returning water to rivers，the sea and the ground.In most areas of China rivers， lakes and groundwater are severely depleted，contaminated and overexploited. The Chinese Government fully recognizes the problems and has put in place policies and laws to improve the situation by drawing on the best experiences of turning around water quality，especially from Europe.But these are complex problems that require massive，longterm investment，as well as a shift in peoples attitude towards the environment.For a long time China has run campaigns to reduce wasteful pollution from agriculture，set discharge standards for industry，close the most polluting enterprises and require the construction of wastewater treatment plants for cities.Such actions have helped but we are still a long way from the ultimate environmental improvement goals and it is difficult to enforce and guarantee a healthy environment.

The safe treatment of wastewater is one of societys most fundamental challenges.We believe that a holistic approach is the most effective in addressing a range of key issues within the water value chain， which are inter-related，in order to minimise any negative impacts on human health and the environment while maximising energy and resource efficiency.We are not able to address all the key issues in this review. Instead，we shall focus on the technical tools available for water quality improvement and the trends in their developments.In particular，attention will be given to techniques for achieving very low concentration of nitrogen （TN） and phosphorus （TP） in water.

2 Historical developments

The history of Wastewater Management may be considered from two different perspectives，the design and engineering of the wastewater collection system on one hand and the development of the treatment process itself on the other.According to Wiesmann et al （2006），one of the most ancient systems of wastewater management was constructed in Mohenjo-Daro near the river Indus （Pakistan） in about 1500 BC.In such systems private and public houses were equipped with toilets.Water used for washing and bathing as well as rain water flowed through special grooves into canals which were built with the necessary slope to transport the wastewater into the river Indus.These installations demonstrate the high hygienic standard of an early culture.The main wastewater collector in Rome，the Cloaca Maxima，followed the course of an old ditch which was used in about 500 BC as a conduit for wastewater.

In Europe by the middle of the 19th century，wastewater produced in the fast-growing industrial regions and cities was discharged directly into canals and rivers，as well as into the soil below the toilets in the courtyards of housing blocks.Frequently， the drinking water pumps would be located close by to the toilets.It was no wonder that waterborne diseases often occurred，especially in large cities.The first comprehensive sewer network in Europe began in 1848 by William Lindley in Hamburg. In London，Bazalgettes construction of the sewer system started in 1858 which was essentially complete by 1865.The London sewer networks were laid out over 83 miles（133km） to drain an area of about 100 square miles （256km2）.

The collection of wastewater in canal systems served a purpose，but its discharge into rivers was not a proper solution to the problem，if the amount of wastewater discharged was too great in comparison with the river flow rate.The answer was activated sludge，a biological treatment process which was discovered in 1913 by Arden and Lockett in the laboratories of the Manchester sewage treatment works.By 1938，the activated sludge process was in operation in hundreds of sewage treatment works and million tons of sewage was treated every day.The activated sludge process and its many variants is now the main engine of secondary sewage treatment and has probably had the biggest impact of all processes upon environmental improvement in the past century.

3 Nutrient removal

Biological denitrification had been known about since the late 1800s，but denitrification was only observed at sewage treatment works in the late 1930s（Edmondson and Goodrich 1947）.They used nitrate as a source of oxygen for an overloaded biological filter.Processes for nutrient removal to help prevent eutrophication and to protect water sources from high nitrate concentrations have developed rapidly.In 1962，in the US， Ludzack and Ettinger put forward the use of anoxic zones to achieve biological denitrification in an activated sludge process.This concept is now standard practice in all activated sludge processes and some fixed film processes.

One of the major problems with activated sludge systems in the period up to the early 1960s was that the oxidation of ammoniacal nitrogen （nitrification） was not reliable or predictable. The solution to this was discovered by Downing et al.（1964） who demonstrated the importance of two principal genera of bacteria，Nitrosomonas and Nitrobacter，in biological nitrification processes.In order to achieve reliable nitrification， two conditions must be fulfilled： first， the sludge age has to be sufficiently high to prevent the washout of the slow-growing nitrifiers， and secondly，the contact time between the bacterial mass and the ammonia must be long enough to oxidize the ammonia.

Phosphorus is of course the most problematic nutrient in water. There are currently two main methods for the removal of soluble orthophosphates from wastewater，chemical phosphorus removal（CPR） or enhanced biological phosphorus removal（EBPR）.CPR involves the precipitation of phosphate out of solution by the addition of a metal salt and removal from the wastewater using a solid liquid separation process such as clarification or filtration. CPR is considered to be a reliable，yet unsustainable process，resulting in an increase in sludge production.EBPR is performed by specialist microorganisms known as phosphorus accumulating organisms.The process was discovered by accident by James Barnard （1974） and his colleagues who observed the removal of phosphorus by a plant which was designed for denitrification.

4 Demands for very for very low nutrient levels in effluent discharges

Although in the last 25 years the EU and the US have seen wastewater discharges from all known sources strictly controlled，the Environmental Authorities there are still far from satisfied with the environmental improvement.Their water quality nutrient criteria will have a significant impact on the water pollution control industry due to stringent TN and TP requirements.

The establishment of stringent TN and TP effluent limits will dramatically increase the capital and operational costs of wastewater treatment plants.Various activated sludge configurations have the ability to consistently achieve effluent TN concentrations between 6-8 mg TN/L and effluent TP concentrations between 0.5-1.0 mg TP/L，without external carbon addition.Typically，however，additional advanced nutrient removal technologies are used in tertiary treatment to meet effluent requirements that are lower than the activated sludge TN and TP removal capacity.

Previously，a worldwide literature search of advanced nutrient removal technologies by Pagilla et al（2006） identified a number of wastewater treatment plants consistently achieving <5mg TN/L and <0.5mg TP/L. They suggested that research and development needs to improve successful technologies to achieve very low TN and TP effluent standards. The dissolved and colloidal organic N have been identified as major challenges in achieving very low levels of TN.Technical and economic challenges to achieve very low TP effluents include alkalinity deficiency， high chemical usage，high sludge production and lack of sufficient influent BOD for biological P uptake.

More recently，in order to identify the technology gaps and research needs to improve successful technologies for the achievement of very low TN and TP effluents，the Water Environment Research Foundation（2011） had carried out a comprehensive study of nutrient removal plants designed and operated to meet very low effluent total nitrogen（TN）and total phosphorus（TP） concentrations.The study found that the treatment capability of existing technologies were limited to 3.0 mg TN/L and 0.1 mg TP/L.However，the study report stated that there was a need to define their capabilities and reliabilities in the real world situation of wastewater treatment plants.There was also a concern over the very low daily permits for ammonia that would cause the treatment works to be extended to include nitrification reliability at a great cost.

5 Technologies to achieve very low nutrient levels

One advance nutrient removal technology that has been successfully implemented in the US to meet stringent TN and TP limits is denitrifying filters （Kaldate et al，2009）.Denitrifying filters are an advanced TN and TP removal technology that are typically implemented when effluent TN requirements are less than 6mg/L.This technology is an attached growth filtration process that grows a biofilm on solid media， which is typically sand or synthetic material.The attached biofilm converts oxidized nitrogen （nitrate and nitrite） to nitrogen gas with the addition of an external carbon source.Additionally，denitrification filters remove suspended solids， which reduce particulate organic nitrogen and phosphorus，and consume soluble phosphorus to perform both TN and TP removal.This“add on” TN removal technology to an existing EBPR activated sludge process provides “two barriers” for TN removal， which is attractive due to seasonal TN removal strategies that can be implemented.

Unfortunately，the current design of denitrification filters is not effective for TP removal and does not offer a guarantee of TP removal performance.However，Lee et al（2015） reported that the City of Waukesha， Wisconsin conducted four separate pilot studies to explore options for meeting upcoming ultra-low total phosphorus（TP） limits in the State of Wisconsin.Their tests showed that the BluePRO and CoMag systems both reliably met an effluent target as low as 0.04 mg TP/L（as a 6-month average）.The BluePRO system uses a continuously backwash gravity sand filters together with chemical dosing to achieve TP removal in a process known as reactive filtration.The CoMag system uses magnetite particles as a weighing agent in a conventional chemical flocculation system to enhance settling rates thereby reducing plant investment costs by limiting the clarification tank size.

NPC is an innovative nutrient capturing process still under development by Reach Green.It is based on the assimilation of nutrients （both N and P）during heterotrophic growth which uses a carbon source for energy.Potentially，any easily biodegradable organic compounds could serve as a useful substrate.However，over the last decade polyhydroxyalkanoates （PHA） have appeared as a promising constant carbon source for denitrification in wastewater treatment （Hiraishi and Khan，2003） and also provides a favourable surface for biofilm development.Further，volatile fatty acids from waste activated sludge fermentation were found to be a suitable feedstock for PHA production （Morgan-Sagastume，et al.，2013） therefore making the use of PHA in wastewater treatment highly sustainable.In recent years，PHA has been used in a heterotrophic growth process as a convenient means for nutrient removal in aquarium （Hovanec，2013）.This novel approach deals with TN and TP removal in a single step rather than the traditional three steps（nitrification， denitrification and precipitation）.According to Koenig & Liu （2001） heterotrophic reaction rate constants are one order of magnitude higher than those of autotrophic（nitrification） thereby reducing the size of the equipment needed and potentially providing a substantial scope for capital cost reduction.The results of the latest laboratory trials by Reach Green suggest that concentrations as low as 0.5mg TN/L and 0.02mg TP/L are readily achievable.The NPC process could potentially provide an excellent solution for tertiary TN and TP removal world wide as well as a new capability for dealing with the nutrient pollution already in the environment.

6 Conclusion

The history of wastewater management is traced back to the ancient sewerage systems of in Mohenjo-Daro near the river Indus in about 1500 BC. Modern sewage treatment began in Europe at the height of the industrial revolution，arising out of the need to prevent the spread of waterborne diseases.As the EU expanded， new Directives were passed for the prevention of water pollution and the protection of cross-border water resources.This has led to more stringent standards for the control of nutrients.In the last 25 years the EU and the US were demanding ever more stringent standards for effluent discharges.The establishment of stringent TN and TP effluent limits will dramatically increase the capital and operational costs of wastewater treatment plants.Various activated sludge configurations have the ability to consistently achieve effluent TN concentrations between 6-8mg TN/L and effluent TP concentrations between 0.5-1.0mg TP/L，without external carbon addition.Typically， however，additional advanced nutrient removal technologies are required when the effluent requirements are lower than the activated sludge TN and TP removal capacity.Denitrifying filters have proved successful in meeting the stringent TN limit，but not the TP limit.Currently，reactive sand filters is the most popular tertiary treatment method used to meet effluent targets below 0.05 mg TP/L. The Reach Green innovative N and P capturing process， which uses a carbon source for nutrient assimilation， has indicated that concentrations as low as 0.5mg TN/L and 0.02mg TP/L are readily achievable. The NPC process could potentially provide an excellent solution for tertiary TN and TP removal world wide as well as a new capability for dealing with the nutrient pollution already in the environment.

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