Wastewater treatment plant monitors Greenhouse Gas emissions!

15/04/2014

Globally, little attention is paid to gaseous emissions from wastewater treatment processes. This contrasts greatly with the regulatory monitoring that is applied to the quality of water emissions from such facilities. However, in Helsinki (FI), a large municipal wastewater treatment facility continuously monitors its emissions of greenhouse gases (GHGs) to help in the city’s efforts to combat climate change and also to help improve the wastewater treatment process.

Employing a multigas FTIR (Fourier Transform InfraRed) analyser from Gasmet, a Helsinki-based manufacturer of analytical instrumentation, the plant’s managers are able to measure the effects of process control on GHG emissions such as carbon dioxide, methane and nitrous oxide. This also provides an insight into the fate of nitrogenous compounds within the wastewater stream.

Background
The Viikinmäki wastewater treatment plant was built in 1994 to process wastewater from both domestic (85%) and industrial (15%) sources. However, the average temperature in Helsinki between December and February is around minus 4 DegC, with extremes below minus 20 and even minus 30 DegC, so the plant was built almost entirely underground to avoid the freezing temperatures. Underground construction is common practice in the Nordic countries, providing other advantages such as land availability above the plant and the provision of stable conditions for process control and odour management.

Viikinmäki Wastewater HSY (FI)

Viikinmäki Wastewater HSY (FI) (Photo courtesy of HSY)

The Viikinmäki plant is the largest wastewater treatment facility in Finland, handling approximately 270,000 m³ of wastewater per day, which amounts to about 100 million m³ per year. The wastewater is treated in compliance with the Finnish Wastewater Discharge Permit, which is stricter than the EU Water Framework Directive for parameters such as nitrogen removal, phosphate content, BOD, COD and suspended solids. Following treatment, the purified / treated wastewater is conveyed 8km out to sea and to a depth of over 20m. This might seem superfluous, but the 16 km long discharge pipe was built in the 1980s and was designed to ensure that discharged wastewater did not accumulate on the shallow and scattered shore and nature reserves along the coastline of Helsinki.

The treatment process is based on the activated sludge method and includes three phases: mechanical, biological and chemical treatment. Traditional nitrogen removal has been enhanced with a biological filter that utilises denitrification bacteria.

The organic matter contained in the sludge produced in the wastewater treatment process is exploited by digesting the sludge, and the biogas generated in the digestion process is collected for further use. Thanks to the energy produced from biogas, the treatment plant is self-sufficient in terms of heating and about 70 per cent self-sufficient in terms of electricity. However, the plant aims to be fully energy self-sufficient in the near future, and around 60,000 tonnes of dried waste sludge is sold each year for landscaping purposes.

Gas monitoring
As a result of the size of the plant (E-PRTR reporting) and the commitment of the Helsinki Region Environmental Services Authority (HSY) to the protection of the environment, it was necessary to monitor or to model gaseous emissions. At the beginning of the E-PRTR reporting requirements (2007) HSY modelled the annual gaseous emissions based on grab samples. However, monitoring was relatively simple to implement because the plant is enclosed underground and a gas exhaust system was already in place.

Viikinmaki Emissions Monitor

Viikinmaki Emissions Monitor (Photo courtesy of Gasmet Technologies)

Initially, a portable FTIR analyzer from Gasmet was hired for a short period to assess the plant’s emissions and for research purposes. However, as Mari Heinonen, Process Manager at Viikinmäki, reports: “The gas emissions data were very interesting but they were not representative of the annual emissions, and posed more questions than they answered.

“We therefore purchased a continuous emissions monitoring system (CEMS) from Gasmet, which was installed in late 2012 and we now have our first full year’s data for 2013.

“Very little data has been published on the GHG emissions of wastewater treatment and as far as we are aware, Viikinmäki is the only plant in the world conducting this type of monitoring, so our data is likely to be of major significance.”

The Gasmet CEMS employs an FTIR spectrometer to obtain infrared spectra from the waste gas stream by first collecting an ‘interferogram’ of the sample signal with an interferometer, which measures all infrared frequencies simultaneously to produce a spectrum from which qualitative and quantitative data are produced. For example, the CEMS at Viikinmäki continuously displays emissions data for CH4, N2O, CO2, NO, NO2, and NH3.

Over a number of years, Gasmet has established a library of FTIR reference spectra that now extends to simultaneous quantification of 50 gases or identification of unknowns from a collection of 5000+ gases. This means that it is possible to reanalyse produced spectra with the instrument’s PC based software (Calcmet) and thereby to identify unknown gases – a major advantage of FTIR.

Whilst FTIR is able to analyse an enormous number of gases, the technique is not suitable for noble gases, homonuclear diatomic gases (e.g., N2, Cl2, H2, F2, etc) or H2S (detection limit too high).

Gasmet FTIR technology was chosen for the Viikinmäki plant because of its ability to monitor multiple gases simultaneously. However Mari Heinonen says: “The system has performed very well, with very little maintenance required. Zero point calibration with nitrogen (background) just takes a few minutes each day and is fully automated. Water vapour calibration is conducted at least once per year, but under normal circumstances no other calibration is necessary.”

With the benefit of the monitoring data, Mari Heinonen has calculated the annual emissions for methane to be around 350 tonnes, and for nitrous oxide around 134 tonnes. This means that the emissions per cubic meter of wastewater equate to 3.5g of methane and 1.34g of nitrous oxide.

Looking forward, Mari believes that it will be possible to use the gas monitoring data to improve process control: “Traditional monitoring/control systems focus on concentrations of oxygen, nitrate and ammonia in the water, but if we detect high levels of N2O gas for example, this may indicate a problem in the process that we can use as a feedback control.

“The monitoring data for gaseous nitrogen compounds (N2O, NH3, NOx) complements water analysis and provides a more complete picture of the nitrogen cycle in the treatment process.

“Clearly, further research will be required, but this work may indicate a need to consider the fate of nitrogenous compounds beyond just those in the wastewater; the removal of nitrogen from wastewater is a key objective, but if this results in high N2O emissions the process may need to be managed in a different way.”


Wastewater treatment optimisation provides cost savings

25/08/2010

Dr Michael Haeck, Hach Lange

Background
The operators of wastewater treatment plants constantly seek new opportunities to improve plant efficiency and environmental performance. In order to achieve this they need to be able to maintain the effectiveness of the treatment process, producing a consistent discharge within consent limits, whilst minimising inputs such as energy, labour and raw materials.

Real-time control (RTC) has become very reliable.

As technology advances new opportunities materialise and this article will outline the considerable benefits that can be obtained from the latest sensors coupled with a new breed of real-time controllers. Improvements in the accuracy and reliability of sensors, coupled with a new facility providing  information about the sensors’ performance, in addition to the measurement itself, means that real-time control (RTC) has become very reliable which means that it has become an attractive option in a large number of applications.

Hach Lange has developed a set of standardised control modules, enabling the application of processes improvements and optimisation strategies without the need for complex programming and expensive customisation.

In combination with Hach Lange sensors, Nutrient Removal and Sludge Treatment Processes can now be easily optimised in order to achieve savings in aeration energy and chemical consumption, even on small waste water treatment facilities.

RTC opportunities
Stand-alone wastewater treatment optimisation solutions (WTOS) control modules are now available to optimise individual treatment processes at treatment plants. These can be easily integrated into an existing plant structure and currently include (1) the chemical elimination of phosphorus and (2) dissolved oxygen adjustment according to the actual NH4-N load in an aeration tank.  Control modules for sludge management as sludge retention time controller or desludging controller will be added in the near future.

In addition to the stand-alone modules mentioned above, it is also possible to combine different RTC modules to optimise an entire plant, as outlined in the trial below. Termed an ‘enterprise solution’ this activity involves a review of the plant as a whole and the creation of customised specifications for the application of different control modules for nitrification, sludge retention time, methanol dosing, and/or chemical phosphate removal to achieve the best overall performance.

Sensor technology
In recent years, improvements in sensor technology have focused on greater resolution and accuracy in combination with longer intervals between calibration or service. However, in order for an RTC system to operate effectively it is also necessary for sensors and analysers to be able to provide information on the quality of the signal and the service status.

Hach Lange has filed a patent application for this facility under the brand name ‘PROGNOSYS’. This provides the RTC control modules with a continuous indication of a sensor’s status so that if pre-determined conditions occur (sensor failure, outside calibration, service overdue, drift etc) the RTC automatically adopts an alternative control strategy, which might be a typical weekly and diurnal flow profile that has been stored in the system’s memory.

Stand-alone RTC example: chemical Phosphate removal
As outlined above, the measurement technology for phosphate has advanced considerably in recent years in tandem with a reduction in capital and operational costs. As a result, an easy to integrate RTC module in the phosphate removal process can deliver pay back periods of less than one year.

The measurement of phosphate levels in combination with an RTC system can be utilised to manage the dosing of precipitant salts. This precipitates the phosphate and facilitates sedimentation and removal. Accurate continuous monitoring is necessary to ensure that (a) sufficient dosing is applied to remove the phosphate and (b) excessive dosing does not take place. Over-dosing would be undesirable on three counts; firstly, from an environmental perspective the objective is to minimise the amount of iron being added that could remain in the effluent; secondly, ferric sulphate is expensive and excessive dosing would be costly; thirdly the amount of precipitation sludge should be kept to a minimum because sludge disposal can represent a significant cost.

A unique feature of the RTC system is the continuous automatic calculation of the ‘ß’ value (overdosing rate), which is required to calculate the right amount of precipitant dosing for open loop control. The calculated ß-value takes into account the percentage of phosphate which has to be removed. The less phosphate there is; the more difficult removal becomes and the more precipitant is required to eliminate the same amount. For example, more precipitant is required to lower phosphate concentrations from 4 to 2 mg/l than from 6 to 4 mg/l.

Wastewater treatment plants operating an open loop real time control system for phosphate removal have demonstrated considerable savings – a UK works has saved approximately 37% of the ferric sulphate cost and 57% of caustic chemical costs and a plant in Italy has shown 50% cost savings in comparison with a constant dosing system, which represents a 7 month payback.

If closed loop control is applied, the RTC system requires a measurement of phosphate levels immediately after dosing. As a result, the Phosphate concentration can be held at a fixed desired level and the control performance is monitored as indicated in figure 1.

Figure 1: Example for Stand Alone P-RTC performance

UK RTC Trial – activated sludge process control
The results of a trial investigating the benefits of an RTC system on the management of the activated sludge process (ASP) have been published by Thornton, Sunner and Haeck[i].

Managed by MWH UK Ltd and employing monitoring instruments from Hach Lange, the trial employed online sensors and control algorithms to optimise the operation of the ASP, leading to greater efficiency and sustainability. Undertaken at full scale, the trial assessed the benefits of RTC at a 250,000 population equivalent (PE) works in the UK and consisted of two identical ASPs (each with four lanes) configured as a 4-stage Bardenpho plant with methanol addition in the secondary anoxic zone.

Standard aeration lanes (fixed DO set-points with fluctuating NH4 effluent concentration) were compared with lanes running an RTC system operating variable DO set-points based on actual load. The RTC lanes deployed extra sensors for dissolved oxygen, ammonium and nitrate.

The trial demonstrated that the RTC system was able to respond quickly to ammonium influent spikes and to maintain a stable effluent ammonium level. The trial also demonstrated that the RTC system was able to reduce methanol consumption by 50% and energy (measured as air flow) by 20% (figure 2). The system has now operated successfully for more than one year

Figure 2: RTC savings

Summary
The Hach Lange optimisation system combines process measurement technology with advanced RTC control modules to provide substantial savings in operational costs at wastewater treatment plants, whilst maintaining compliance with consent values.

Recent advances in sensors, analysers and controllers mean that wastewater treatment no longer has to be managed on a ‘worst case scenario’ basis. Processes can now be monitored and adjusted instantaneously to maximise efficiency and improve process stability. Cost reduction is obviously a key benefit, but the ability to reduce energy consumption is becoming an important objective in many countries.


[i] Thornton, Sunner and Haeck, 2010. Real time control for reduced aeration and chemical consumption: a full scale study. Water Sci. Technol.61, 2169–2175