The Lille metropolitan area awarded Veolia Water the contract to reconstruct the Marquette-lez-Lille wastewater treatment plant (WWTP) in 2010, the biggest such facility in the north of France. The capacity of the new WWTP is 700,000 m³/day, serving a population of 620,000. It will be built on the site of the previous one, which is now obsolete.
The renovation work comprises four treatment lines, including wastewater, stormwater sludge treatment lines, and a deodorization line. By applying state-of-the-art design and proven technologies, especially Hybas™ and Exelys™-DLD, a sustainable development solution was provided to local communities.
The effluent from the new plant will meet the requirements of the European Water Framework Directive (Directive 2000/60/EC) on ‘good ecological status’ of water, and the treated storm water will be used to irrigate 7 hectares wetland. Compared with conventional digestion, the Exelys™-DLD configuration will further reduce sludge quantities by 30% and increase biogas production by 20%. Thereby, 94% of the power requirements will be supplied from the plant itself.
The water line started operating in February 2013, and commissioning of the sludge line, which started in July 2014, was expected to finish in February 2015. In this paper, the design concept of the sustainable development solution is presented, as well as the application of showcase technologies for wastewater and sludge treatment.
The Lille Metropolitan Area is in Northern France. It covers 61,000 hectares and has a population over one million. The old Marquette-Lez-Lille Wastewater Treatment Plant (WWTP), built in 1969, served local residents for more than 40 years but was no longer meeting current environmental standards, particularly in terms of nitrogen and phosphorus removal. Because of this, it became necessary to upgrade this facility in the area, in a sustainable and efficient manner.
Veolia Water was awarded the contract to reconstruct the Marquette-lez-Lille WWTP in 2010, including designing and building the plant, administration building, and landscaped areas, as well as 5.5 years of operation. The new treatment plant is built on the site of the previous one, which is now obsolete, and will meet the requirements of the European Water Framework Directive (Directive 2000/60/EC) on ‘good ecological status' of water.
There are two separate treatment trains in the water line in the new plant, one for wastewater (2.8 m3/s) and the other for stormwater (5.3 m3/s). Stormwater will be treated using the patented Actiflo® ballasted flocculation process, while the wastewater will be treated successively by a series of proven technical solutions:
Multiflo™, a lamella settler used as the primary treatment for suspended solids removal;
Hybas™, a biological treatment hybrid technology combining the best of two well- known technologies: activated sludge and AnoxKaldnes MBBR; and
Hydrotech Discfilters™ used as tertiary treatment to polish the effluent.
For sludge treatment, the implementation of Exelys™, a new thermal hydrolysis process (THP) from Veolia, will reduce the quantity of sludge produced by 20–40% and increase the production of biogas by 15–30%, compared to standard digestion. After being dried in a BioCon™ dryer and stored, half of the sludge will be used in agriculture and the other half in a cement works.
As the WWTP is located in a densely populated urban area, particular care has also been taken in dealing with odors. Full control over odor emissions, treatment, and monitoring is provided by the dedicated odor treatment lines to minimize nuisance.
By integrating the most compact solutions, reducing the use of fossil fuels and generating energy, a sustainable approach was achieved to turn waste to energy and reduce the plant's carbon footprint.
The new WWTP will have the capacity to treat wastewater from an equivalent population of 620,000. The process scheme and layout of the new treatment line are shown in Figure 1.
Both wastewater and stormwater are received at the inlet chamber, which is equipped with gas detectors and isolation valves, to avoid any risk of accumulation of dangerous gas or explosion. Four (4) sets of coarse screens are installed to prevent ingress by elements larger than 25 mm. This is the only pumping station in the plant, which helps to limit energy consumption. The stormwater and wastewater streams are also divided here. The overflow exceeding 2.8 m3/s will be directed to the stormwater treatment line.
From the pumping station, the raw water passes through six (6) sets of fine screens (6 mm) to protect the downstream process units. The pretreatment is done by four (4) longitudinal grit and grease removal chambers, to remove sand and fats from the wastewater.
Subsequent primary decantation is carried out using Multiflo™, Veolia's compact lamella settler technology, which works without chemical dosing and removes most of the suspended solids (Figure 2).
The heart of the wastewater treatment system is the removal of dissolved pollutants, essentially carbon, nitrogen, and phosphorus. This is done biologically in three tanks associated with six clarifiers, using the Hybas™ process. It operates with a mixed bacterial culture, some of which moves freely in the tanks while the remainder forms a fixed biofilm on submerged media. The role of the bacteria is to transform the dissolved pollutants into substances that will precipitate and can be separated as a biological sludge from the treated water in the clarifiers. These are of 49 m diameter and 4.5 m deep. The clarifiers are equipped with photovoltaic cells on the roofs for power generation.
Particulate phosphorus remaining on the suspended solids at this stage is removed by a 10 μm sieve Discfilter™. At the Discfilter™ outlet, a portion of the treated water is diverted into the distribution system to be reused for the preparation of polymer solution, the irrigation of green spaces in the community and the jet cleaning of the plant facilities.
The effluent standards required for the new WWTP are listed in Table 1.
|Parameters .||Requirements of DCE 2013 (mg/L) .||Veolia guarantee value (mg/L) .|
|Parameters .||Requirements of DCE 2013 (mg/L) .||Veolia guarantee value (mg/L) .|
The feed to the sludge line includes the primary and biological sludges and that from the stormwater line. There are three sludge treatment steps. The design of the sludge treatment lines is intended to maximize biogas production and minimize the residual amount of sludge to be treated, as shown in Figure 3.
The primary and biological sludges are sent, with about 70% of the stormwater sludge, to the primary digester, where biogas is produced. This is used in the plant, as well as yielding biofuel. The digested sludge is dewatered by centrifugation and sent on to the second stage. Here, it is subject to thermal hydrolysis, to make it more easily biodegradable.
A further digestion stage is proposed, following thermal hydrolysis. The residence time is short, compared to normal digestion, but organic matter degrades and is released as biogas, yielding more biofuel and producing electricity by co-generation.
After digestion, the sludge is dried on two belt-dryers, which have a unit evaporation capacity of 2t/h. The dried sludge can be used in agriculture or cement manufacture.
Before being used in agriculture, the dried sludge must be quarantined for a month, pending the return of laboratory test results. This is done in 4 silos, each holding 260 m3, and supplemented by 8 further silos offering total on-site storage of 3 months.
To ensure the reliability of agricultural application, sludge derived from stormwater during heavy rain is managed separately, as it is likely to be polluted and could thus contaminate all of the material produced, leading to its prohibition for agricultural application. The sludge arising from heavy rainfall (about 30% of the stormwater sludge) is limed, stored and disposed of, either in cement manufacture or to landfill.
PRINCIPLE AND DESIGN OF HYBAS™ AND EXELYS™ UNITS
The core treatment steps of the Marquette-Lez-Lille WWTP are the Hybas™ and Exelys™- DLD processes, which play the key roles.
Hybas™ biological treatment
Hybas™ is an integrated fixed-film activated sludge process, combining the best properties of two well-known technologies, i.e., common activated sludge (CAS) and MBBR. It is a biological treatment for carbon and nitrogen. It combines free biomass, which evolves freely in the reactor, as in CAS, with fixed biomass, growing as biofilm on carriers suspended in the tank.
Hybas™ can be a cost-effective means of upgrading an existing activated sludge system to induce or improve nitrogen and/or COD/BOD removal. By adding carriers to the aerated activated sludge volume, nitrification capacity can be increased within the existing volume because the nitrification bacteria grow on the carriers. BOD removal and denitrification occur in the suspended phase. The long sludge retention time, which is normally needed in CAS for nitrification, can be reduced because of the biofilm solution. In addition, reduction of the sludge concentration decreases the sludge load in the secondary clarifiers. The shorter sludge age often reduces the risk of growth of filamentous bacteria and, generally, biological activity is higher in an AS process with low sludge age than in one with older sludge.
Hybas™ also enables an equal load reduction of the biological reactor volume, which improves sludge settlability, avoiding the development of filamentous species and reducing sludge leakage risk in the treated effluent. The micro-organism division between the free and fixed biomass facilitates nitrogen treatment, even at low temperatures, which becomes more stable than in a traditional AS process with respect to load and temperature variations, as well as being easier to operate (Figure 4).
In the reconstruction works at Marquette-Lez-Lille, Hybas™ was selected for all of the reasons given above. The process is used in three (3) parallel treatment lines, comprising pre-anoxic, anaerobic, and denitrification zones, using Hybas™ in the nitrification, post denitrification and post aeration zones (Figure 5).
Factors that affect the overall design of an upgrade include the media type, aeration system, operational dissolved oxygen concentration, and effluent NH3-N concentration, as well as the tank configuration and hydraulic profiles. All are essential factors in creating a treatment system that works well. Should any of them not be considered and/or designed properly, the treatment system can be adversely affected.
The new generation carrier Bio-Chip M was used in the Hybas™ reactors, as they provide a high protected surface area, up to 1,200 m2/m3. About 1,250 tonnes (equal to 7,500 m3) of the carriers were used.
The main advantages brought by Hybas™ are:
▪ Compact – growth of nitrifiers or other bacteria requiring a longer sludge age on carriers means lower AS volume requirement.
▪ A smaller amount of carriers is needed than in standard MBBR™.
▪ The sedimentation characteristics of the sludge are improved because of the reduced sludge age in the AS process.
▪ The biomass is specialized, so both the activated sludge and biofilm are highly efficient.
Biogas yield enhancement is generally achieved through pretreatment. Thermal hydrolysis was first used to improve sludge dewaterability. It broke down the sludge structure by breaking the bacterial cell walls, thereby releasing the highly biodegradable cellular liquor. This increases the partial solubilization of sludge, which enhances anaerobic digestion, as can be seen in numerous studies on thermal hydrolysis for pretreatment for anaerobic digestion (Carrèrea et al. 2010).
Thermal hydrolysis has traditionally been performed in batch systems – e.g., some commercialized industrial processes such as Cambi and BioThelys™. Although these are proven and effective, the batch system involves high capital costs and energy requirements, and complex system operation, which limited their application. ‘Batch to continuous’ is a normal process evolution, giving a smaller footprint, simpler control, and easier servicing and maintenance (Stedman 2010). Exelys™, developed by Veolia, improves thermal hydrolysis (THP) from batch to continuous operation, and offers an alternative solution for the energy sufficiency of WWTPs.
The Exelys™ process is designed for continuous operation, and handles biosolids with dry solids (DS) content greater than 25% w/w at 165 °C and 9 × 105 Pa. It is shown in Figure 6. Different configurations of Exelys™, such as ‘digestion-lysis’ (DL) and ‘digestion-lysis-digestion’ (DLD), have been developed to enhance anaerobic digestion capability, by applying Exelys™ as the sludge THP. Exelys™ – DLD is used in the Marquette-Lez-Lille WWTP.
The Exelys™-DLD system comprises 3 steps –digestion, thermal hydrolysis, and digestion.
Step 1: Digestion – centrifuge dewatering
The biological sludge is predigested, and then dewatered by centrifugation.
Two (2) mesophilic digesters of unit volume 6,100 m3 are used for the primary digestion. The sludge retention time at the weekly sludge load is 12 days, at 36 °C. Based on the 64.6% volatile matter ratio, the MS (solid material) reduction ratio could reach 28% in the primary digestion, yielding more than 10,000 Nm3/d biogas.
Four (4) sets of centrifuges are used to dewater the digested sludge; they operate continuously and the unit capacity is 10.6 m3/h. The dewatered sludge reaches up to 22% DS before entering hydrolysis.
Step 2: Sludge thermal hydrolysis
Continuous thermal hydrolysis is carried out in a high temperature reactor (165 °C) at high pressure (8 × 105 Pa).
Four (4) lines of centrifuges transfer the dewatered sludge to intermediate holding hoppers (unit volume 3 m3), each equipped with a feed screw pump. The hoppers also provide a buffer between the centrifuge and the hydrolysis reactor, and allow regular variation. Sensors monitor levels in the hoppers.
The reactor (unit volume 2.6 m3) is equipped to maintain a steady pressure of 8 × 105 Pa, in the reactor, which is completely insulated. Other sensors continuously monitor the pressure and temperature at various points within it. A rupture disc and small surface tank are designed for security, to enable the reactor to be drained completely if the level in it exceeds the set point of the rupture disk, in order to avoid any risk of overpressure in the reactor.
The required steam will be produced partially from the cogeneration system, and partially from a steam boiler used as a backup (Figure 7).
Step 3: Turbo-digestion
The thermally hydrolyzed sludge leaves the reactor at 165° C, and is cooled by dilution and a tubular heat exchanger (water/sludge) before entering turbo-digestion.
The output sludge from the hydrolysis reactor contains 17% DS. To avoid the risk of clogging the tubular exchangers, water is injected into the outlet of each reactor, to reduce the DS content to 11%. Gray water or supernatant from pre-dewatering can be used.
Two heat exchangers are provided per line, to ensure that the hydrolyzed sludge temperature is suitable for Step 2 digestion. After dilution and the first stage heat exchanger, the sludge temperature is reduced from 165 to 50 °C, and a second stage brings it down a few degrees further to 40–42 °C.
Another four (4) mesophilic digesters (unit volume 6,100 m3) are used as turbo-digesters. With hydrolyzed sludge, turbo-digestion operates on a lower sludge retention time (16 days) and biogas production is improved (9,287 Nm3/d). Using hydrolyzed sludge, the MS reduction ratio can reach 44% in turbo-digestion.
The quantity of biogas produced by the primary- and turbo- digesters is up to 19,388 Nm3/d, from the weekly sludge load, and the biogas will be stored in a pair of identical gasholders (unit volume 3,500 m3).
Exelys™- DLD brings several advantages:
▪ Reducing the sludge quantity combined with improved sludge biodegradability. MS abatement and MV reduction is superior to that of conventional digestion. The depression of MS is up to 60%, and MV reduction is up to 30–35%.
▪ Hydrolysis is a means of sludge disinfection: Indeed, as it is heated to 165 °C at 8 × 105 Pa for 1 hour, all pathogenic micro-organisms are destroyed.
▪ Hydrolysis improves the acceptability of sludge for agricultural application and the sludge no longer smells. The sludge is drier (by 3%) and stacks better.
In summary, compared to conventional digestion, the Exelys™-DLD process reduces dry matter by about 20% and wet material by about 40%. Biogas production is increased by 15–20% at the same time. The system will also produce 4,214 MWh/year by cogeneration, after providing all of the heat required for thermal drying.
CONSIDERATION OF THE SURROUNDING ENVIRONMENT
Located on a 15 hectare island surrounded by the Roubaix canal, the plant also offers the opportunity of creating a major environmental project for the Lille area (Figure 8).
A large, landscaped area hosting sporting and cultural activities will be created around the plant, because of the use of compact technologies, like Multiflo™, Discfilter™, and Actiflo®. This will facilitate integration of the plant into the landscape, without inhibiting urban change for improved quality of life.
The layout of the civil engineering structures leaves seven hectares of land available for a garden, which will be planted mainly with local species to reflect the area's biodiversity. To symbolize this, the entrance will open onto two facades designed by Patrick Blanc, the ‘artist botanist’ who invented the vertical garden concept and has designed hundreds of such projects around the world. The architecture will also be designed to blend harmoniously into its environmental and urban surroundings, and will minimize pollution and nuisance for the nearby apartment blocks.
Building a new city-center WWTP to meet the growing needs of the population, comply with European standards and cater for sustainable development was a challenge set to Veolia by the Marquette-lez-Lille WWTP.
The processes selected provide efficient and effective protection to environmental equilibrium. The Hybas™ process combines the good performance of fixed biomass technology and simplicity of activated sludge to ensure the correct treatment levels. Clarification and Discfilter™ as the tertiary treatment provide the final physical barrier, which should guarantee environment protection.
The energy issue was solved by using ‘low consumption’ buildings, reagent free technologies and systematic implementation of heat recovery. Exelys™-DLD thermal hydrolysis contributes by reducing the quantity of sludge and increasing biogas yield, to give more green power.
By taking ‘zero pollution’ as a 100% responsibility, several key features can be achieved using state-of-the-art design:
▪ Meeting the most stringent European discharge standards, i.e.: COD ≤ 90 mg/L, BOD ≤ 20 mg/L, NH4-N ≤ 5 mg/L, TN ≤ 10 mg/L, TP ≤ 1 mg/L;
▪ The footprint of the new plant is one third that of its predecessor, releasing 7 hectares for landscaping, etc.;
▪ The plant is 94% self-sufficient in power, and earns about 500,000–600,000€/yr selling electricity to the national grid;
▪ Treated stormwater is recovered and will be used to irrigate the 7 hectare wetland;
▪ Some 31,000 kWh per year of electricity is produced by photovoltaic panels
The new Marquette-Lez-Lille WWTP transforms the concept of wastewater treatment into an issue of real local pride, involving innovative design and technologies. The future plant will be part of a program to evolve toward sustainable development solutions, setting a good example of a new concept, a pathway towards energy self-sufficient plants with low carbon footprints, and a good showcase for the city of the future.