Articles & Case Studies


Posted: Tuesday 26th May 2009

Monza is world famous as the home of Italian Formula 1 in Milan, but it is also soon to be home to a new sewage sludge processing facility designed and built by SIBA S.p.A (a Veolia Water Solutions & Technologies Business Unit). The € 6.5m plant, which will be built at the S. Rocco wastewater treatment works in Monza, will use Veolia’s Biothelys™ thermal hydrolysis technology and will be operational by June 2010.

The S. Rocco works (Fig 1) is run by ALSI S.p.A, who operate and maintain wastewater treatment plants for the Milan Province. It normally treats sewage from a population equivalent of 690,000 but will increase, in the long term to 750,000 with the planned upgrade of the sewage works. The works generates about 23 tonnes per day (as dry solids) of primary sludge and 20 tonnes per day of surplus activated sludge, which is currently treated by three digesters: two primary digesters of 7,000m3 and one secondary of 2,800m3. ALSI’s objectives were to reduce the quantity of sludge for final disposal by improved destruction of volatile suspended solids and to minimise odours from the works.

The Biothelys™ thermal hydrolysis process is the result of a decade’s development work by Veolia Water Solutions & Technologies, and is used to pre-treat thickened or dewatered sludges before biological digestion. The objective of the process is to hydrolyse organic solids to solubilise them and make them more readily biodegradable, improving the performance of the digester in removing volatile suspended solids. It also reduces the hydraulic retention time required in the downstream anaerobic digester which can, therefore, process more sludge per unit volume of capacity. In the case of a new-build, the digester is smaller and lower in capital cost: in a retrofit the throughput can be increased. Hydrolysis disrupts cellular material, floc particles and organic macromolecules, so the pre-treated sludge is less viscous and easier to pump.

In addition, the greater biodegradability of hydrolysed sludge improves removal of volatile suspended solids which increases the biogas yield and reduces the quantity of sludge for final disposal. Biothelys™ also gives a significant reduction in odour generation.

Thermal hydrolysis takes place at a temperature around 165°C, so the first step of treatment is to dewater the raw sludge to about 15-16% dry solids using a centrifuge or belt-filter in order to minimise the energy required for heating. The Biothelys™ system, shown schematically in Fig 2, consists of two or more batch hydrolysis reactors working in parallel and out of phase with each other.

Each reactor in turn goes through a multi-step cycle taking between 150 to 165 minutes depending on the configuration. First the reactor is filled with raw sludge and this is pre-heated with recycled flash steam from the other reactor. Heating to hydrolysis temperature is completed by injecting live steam into the sludge from a steam generator fired with biogas. Once hydrolysis temperature has been reached it is maintained for a pre-set time before the steam is released as flash steam, which is recovered for pre-heating another reactor. Finally the reactor is emptied using the residual pressure in the reactor to aid discharge to the buffer tank where the hydrolysed sludge is stored prior to being cooled and transferred to the anaerobic digester.

In order to minimize the energy needed for the thermal hydrolysis, different heat recovery techniques can be implemented in the whole process:

Firstly the heat released during the sludge cooling prior to the digester is kept under the form of hot water at around 85ºC. Part of this hot water is then used to dilute the sludge coming into the Biothelys™ reactors (from around 22%DS to around 15-16% DS). This enables preheating of sludge (up to 37ºC) before the thermal hydrolysis and hence reduce the total steam amount needed for the heating.

The second heat recovery is carried out from the CHP engines. The exhaust fumes are released from the CHP engines at a temperature of around 450-500ºC. These hot gases are introduced to a waste heat boiler to produce steam by cooling the fumes to around 230ºC. The steam produced from the waste heat boiler will then form part of the total steam amount needed for the thermal hydrolysis. This again reduces the amount of gas (biogas or natural gas) needed for the steam production. The CHP cooling water is also passed through the boiler thus minimising the amount of fuel required for steam production.

In this changing economic climate some sludge disposal routes have become very expensive or even non-viable, whilst biogas is now a cheaper fuel than natural gas or oil. Therefore thermal hydrolysis is an economically attractive solution due to the reduced sludge volumes and improved biogas generation.

The Biothelys™ plant at S. Rocco will treat 35% of the primary sludge and all the secondary sludge, a total of 28 tonnes per day as dry solids using only one 7,000m3 digester, in two streams each consisting of two 12.5m3 thermal hydrolysis reactors.

The remaining 15 tonnes per day of primary sludge will be thickened in the works’ existing mechanical thickeners and then mixed with the hydrolysed sludge from the Biothelys™ plant prior to digestion in the existing 7,000m3 anaerobic digester. The overall result will be a reduction of about 70% in the volatile suspended solids content of the sludge, with most of that being converted into biogas. The Biogas produced from the process, is used to operate the endothermic engines which in turn operate the air compressors that feed the air to the activated sludge reactors within the wastewater treatment line. The majority of the sludge is dried and then burned in cement production furnaces.

As well as the Biothelys™ process, the improvements to the S. Rocco works includes other processes from Veolia Water Solutions & Technologies. Water from the sludge thickening plant will be treated by chemically assisted sedimentation using the well proven Actiflo™ ballasted floc process. In this small footprint process, microsand is introduced along with coagulant chemicals to provide a seed for floc formation. The resulting flocs are heavy and settle at rates up to twenty times faster than conventional systems. The sand is recovered for reuse via a hydrocyclone that also concentrates the sludge giving a smaller volume for final disposal. Recovered supernatant is returned to the head of the works.

Monitoring of odours on the site will use another Veolia technology: the OdoWatch™ “electronic nose”. Originally developed at the Engineering School of the École Polytechnique de Montréal, OdoWatch™ uses a matrix of sensors to analyse an air sample for a range of parameters, and can be calibrated to respond in exactly the same way as the human nose to identify a perceived odour. This means that it can distinguish between odours generated by a particular works and those originating from other sources. Data from a number of electronic noses – typically between two and five depending on the size of the works – provide continuous updates to a real-time model which merges them with data collected from the network’s weather station to produce an “odour plume”. This is overlaid onto a map of the site and surrounding area. The technology allows prediction of odour impact up to 24 hours ahead, is colour-coded for odour concentration, and the computer issues alerts if preset thresholds are exceeded, making it easy for the operator to assess the odour impact and, if necessary, to take steps to attenuate or eliminate the odour nuisance. The system also identifies the main sources of odour, calculates odour emission rates and provides a data archive in case of subsequent odour complaints by the public.

The new facilities being constructed by Veolia Water Solutions & Technologies at S. Rocco will give about 30% reduction in the sludge volume for final disposal, enhanced biogas production and improved odour management. All of which makes it a solution which is both cost effective and environmentally friendly.

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