Large quantities of effluent can sometimes lead to saturation of water treatment plants, which can no longer handle the excess pollution. To remain within water quality standards, the community or factory concerned must increase the treatment capacity of the plant. Several methods exist: optimisation of existing treatment facilities, better aeration or even oxygenation and use of membrane bioreactors are just some of the approaches to be explored before considering the construction of a new plant.
It is no easy matter to treat wastewater correctly when confronted with substantial variations in pollution load. These are most frequent during holiday periods, heavy rainfall or seasonal industrial activity. The problem is particularly acute for treatment plants dating from the 1960s or 70s, since they were not normally designed to handle stormwater. These plants often need updating in order to meet regulatory requirements and handle the arrival of excess pollution.
Updating implies a total rethink of the treatment system to increase its capacity to handle raised levels of effluent, while taking into account the new regulatory restrictions that make it compulsory to treat nitrogen, phosphorus and microorganisms. The community has a choice between two alternatives, refurbishing the old plant or building a new one.
While starting again from scratch could well be the best solution it is nevertheless the most expensive. Furthermore, in an urban or suburban zone, lack of space is a problem and moving the treatment works to another site is not always straightforward.
Faced with these arguments, a possible solution might be to refurbish the site and convert to more compact equipment. This would allow increased volumes to be treated while minimising nuisance. The site and the collection network remain unchanged. This means either adapting the equipment to boost its treatment capacity or almost entirely rebuilding the system. Alain Rousse, at OTV, is convinced: “Refurbishment will continue to be a growth market”. Moreover, the advent of membrane techniques allows improvements to be made in the level of
treatment without having to alter the site. What is the best solution? Before taking any technological decisions, a detailed review of the situation is required.
A detailed review of the existing situation
A detailed functional review and qualified inspection of the installations (including engineering work) is an essential first step. “Before a plant is refurbished we must check that it is in perfect working order”, says Pierre Gilles, head of the industrial cluster department of the technical branch of OTV. Civil engineering systems must therefore be fully operational, which is rarely the case after they have been in service for 30 or 40 years. If this is not so, it could interfere with any work being undertaken, and ultimately make it difficult to guarantee the effectiveness of the refurbishing work. Another aim of the review is to determine the nature of the excess flow that has to be treated. For this, a number of questions have to be answered. Do peak periods of excessive flow last a matter of hours, days or even weeks? If so, where does it come from? Rain water? A holiday resort? An industrial zone? Beyond a few weeks or even after several days the excess should not be considered as an exceptional occurrence. For Bruno Tisserand, a residual water specialist at the technical branch of Générale des Eaux: “You must then take into account the limiting factors within the entire installation, in particular those relating to the hydraulic and sludge treatment systems”.
In the case of industrial effluents, it is important to carry out an additional audit. If this is correctly done, flows can be quantified and an analysis made of how well the existing pollution treatment system functions. This information is helpful in determining the most suitable oxidation techniques for specific effluents. The audit can be complemented by laboratory tests aimed at verifying the feasibility of the technology chosen following the initial study.
Until quite recently, the first action was to improve aeration in the tanks.
Improving aeration in the tanks
Bacteria in the activated sludge tanks feed on the pollution which they transform. To perform this function correctly, the biomass needs an oxygen-rich medium. In traditional treatment plants, oxygen present in the air is used to aerate the biological tank. The oxygen is provided by aeration systems: air spargers, injectors, turbines, etc.
However, air has a low oxygen content. A considerable volume has to be injected to reach the dissolved oxygen concentrations needed for effective treatment. The performance of the system depends on its oxygenation capacity.
This is the basic criterion for any aerator (or oxygenator), and is determined in the same way in virtually every country. It therefore represents an international reference. It involves measuring the rate of reoxygenation of clear water. To carry it out, a test basin is filled with drinking water. Before the test, the water is saturated with oxygen using the system to be tested. Once the water is saturated, the measuring sensors are calibrated. The level of dissolved oxygen is reduced to zero by the addition of sodium sulphite and cobalt chloride. The basin, which no longer contains any dissolved oxygen, is then reoxygenated until saturation is reached using the material under test. The dissolved oxygen concentration is then measured over time. The migration from the test values in tap water to the values observed in rated conditions requires corrections to be made. The temperature of the water and the absolute pressure have to be taken into account.
If one knows the nominal oxygenation capa-
City it is possible to define the other criteria, such as hourly oxygen transfer capacity, specific oxygen transfer capacity, transfer efficiency, the alpha coefficient, in other words the pure water/liquor equivalency factor. This parameter depends on the nature of the water and in particular on the concentration of matter in suspension, fat, surfactants, etc. It should also be noted that mixing during aeration plays an important role. It promotes the homogenisation of the medium. In these aeration systems O₂ is dissolved at the rate of 1.2 to 3 kg/kWh.
As air contains only 21 % oxygen, the exchange can be optimised by using pure oxygen. This increases the efficiency of oxygen transfer.
Using oxygen for increased efficiency
“Compared to air-based techniques, the use of pure oxygen considerably increases the amount of dissolved oxygen in the water”, explains Étienne Thomas, in charge of nation-wide industrial environment development at Air Liquide. “The transfer kinetics are faster than with air, allowing greater flexibility of use and making it easier to adjust to variations in load by rapidly responding to the biomass requirements”. Two main types of technology currently hold the spot on this market. These are Venturi-type injectors and bubble diffusers. All gas producers, such as Air Liquide, Air Products, Dresser, Praxair and Messer, offer this type of approach.
Praxair, for example, has developed the Mixflo™ system which uses Venturi-type liquid-liquid injectors. Its oxygenation efficiency is greater than 90 %, with a transfer rate of up to 5 kg dissolved oxygen/kWh. The size and position of the injectors are designed to optimise liquid–oxygen mixing. The unit’s contact loop increases the efficiency of oxygen transfer. The injection capacity and power can be adapted to suit changing needs. This equipment includes a feed pump which can be submerged or placed outside the basin.
The same manufacturer still offers the In-Situ Oxygenator, known as I-SO™, an oxygen diffuser system based on the principle of oxygen injection. This equipment can achieve an oxygenation efficiency in excess of 90 % with a transfer rate of 10 kg dissolved oxygen/kWh. The I-SO™ uses a turbine pump. Low-pressure oxygen is fed through the turbine intake below the turbine cap. The high shear force of the turbine transfers the oxygen mass into the water. Undissolved oxygen bubbles escaping from the flow through the cap duct are recycled into the turbine intake. This clever feature reduces the overall volume of oxygen required.
The choice of system depends on the particular constraints of each plant. At Praxair they point out that “the I-SO is easy to install in basins with a depth of over 4 metres. It adapts to changing needs. For example, to increase the oxygen transfer capacity, you only have to increase the rotation speed of the turbine by changing the reduction gear-box”. The Mixflo™ can be adapted to any type of installation or tank, whether it is square, rectangular, circular, ring-shaped, covered or open, new or existing.
While oxygen doping can solve the problem of pollution peaks, it has the disadvantage of being expensive, unless of course it can be used in synergy with an industrial process or an ozonation treatment stage using oxygen.
Oxygen doping: playing the synergy card
Is oxygen doping too expensive? Not necessarily, particularly for the treatment of fluctuating loads from industrial sources. Many factories already use pure oxygen in their industrial processing. For them, using several tons per day to reduce the COD may be an economically feasible solution. Air Liquide, who are very active in the industrial sector, are seeking to develop this synergy. “Most of our installations are in the industrial sector”, explains Étienne Thomas. “For us, a typical installation would be in the agri-foodstuffs sector. The treatment of 5 to 10 tonnes of COD daily is carried out by conventional doping of 1 to 2 tonnes of oxygen”. According to Air Liquide, for units that do not already have oxygen on site, “a supply of oxygen is justified when the daily requirement exceeds 5 tonnes”.
However, industrial operators will need to conduct a very careful analysis. Goodyear Chemicals, which has been equipped with an Air Products system for several years, estimated three years ago that the annual operating costs were 1.21 kF with oxygen compared to 1.63 kF previously (see box). “In this case, the economic balance tilts in favour of oxygenation”.
“For municipal applications, it can be extremely useful when there are considerable variations in effluent”, adds Didier Marchand, in charge of nation-wide water treatment development at Air Liquide. This is the case with the Lavelanet wastewater treatment plant in France. Built by Degrémont, the plant receives concentrated municipal and industrial effluents, which are treated with a highly innovative method of oxygen doping. It involves doping the tanks with the excess oxygen from the ozonation towers. The gas is recovered and pressurised.
downs. The solution is therefore to adopt a global approach.
Dealing with the problem globally
Beyond a certain point it becomes impossible to improvise. An overall view is needed as there are limiting factors in the short, medium and long-term.
To simulate requirements and the problems arising from excess flow, water treatment operators use simulation tools to construct different scenarios covering possible extreme occurrences. This will enable them to control the hydraulic behaviour of treatment plants during periods of increased activity.
Simulation studies have among other things demonstrated the important role played by correctly dimensioned equipment during periods of hydraulic overload.
For example, during the pretreatment stage, grit and fat need to be removed from the effluent at station inflow. In the case of stormwater, not only does it have a high grit and fat content but the volume that has to be treated is very much greater. A study carried out at Arras (France) showed that during rainy weather the pollutant material carried by the network in a single day was equivalent to a week’s sludge production from wastewater treated in the plant. The rate of sludge removal must therefore be adapted to the speed with which the equipment becomes overloaded. This implies a need for more operations, additional personnel, and thus increased costs.
To avoid the equipment becoming clogged with grit carried by stormwater, a grit separator can be used.
For this operation, the German firm Strate has developed the Rundsandfang range of devices.
Before being reinjected into a Ventoxal, a Venturi-type oxygenator developed by Air Liquide, effluents can be treated with pure oxygen. This approach is also advocated by Lionel Rabin of Air Products: “Our assessment centre in biological treatment deals mainly with industrial effluents, but is sometimes called upon to treat mixed effluents”. Air Products recently supplied the oxygenation unit for a mixed wastewater treatment plant handling effluents from a milk processing factory near Tarbes, in the south-west of France.
While oxygen can give a boost to the treatment of unstable pollutants, it cannot solve all the problems. The fact that oxygenation promotes a more active biomass and therefore more growth inevitably means an increase in the volume of sludge.
Beware of excess volumes of sludge
While some manufacturers, such as Praxair, who state that “experience shows that oxygen can double the speed of sludge sedimentation, thus doubling the flow into the clarifier and reducing filtration time”, others such as Anjou Recherches emphasise that “the more effective the bacteria, the more they reproduce. The treatment is improved but produces more sludge. This can lead to separation problems and the risk of destabilising the fauna, a factor which needs to be controlled”. One is then faced with a limiting hydraulic factor: “What is the maximum quantity of sludge that the plant can effectively separate?”
Of course, one can always use ploys such as injection of polymers, clay, talc, etc., to improve settling. But whenever 30 to 50 % mass equivalent is added, it makes a lot more sludge. “With this approach, the capacity of the plant will very quickly be reached”, explains Bruno Tisserand, “this solution can be adopted for several hours or days, but not weeks, because there is a very great risk of blocking sludge removal”.
Producing more sludge means being able to treat it, in other words dewatering and storing it. Each stage must be suitably adapted to handle a surge.
It is of course possible to recycle sludge during a surge, but this overloads the basins and can destabilise the biomass. It will then take from two weeks to a month for the plant to return to normal. Coping effectively with a surge will thus be at the expense of operation in the medium or long-term. Excessive capacity reduces the room for manoeuvre and increases the risk of breakdowns.
Circular grit separators, which allow 95 % of grit with a diameter of less than 0.2 mm to be removed, at 80 % of maximal flow, reach 1,000 m³/h on the SKPK model. “The middle influent well, in which the grit-carrying wastewater arrives, increases the accelerations to which the water is subjected,” explains Benoît Gartner of Strate, “facilitating the separation of solids contained in the water.”
Additionally, two compressed-air injection stirring swirls boost the upwelling of hydrated particles to the surface, thus avoiding the need to over-dimension grit washers. Hydraulic studies have once again highlighted the important role played by clarifiers during periods of peak flow.
The major role of clarifiers
“The generation of plants in need of refurbishing dates from the 1960s and 70s,” explains Pierre Gilles. “The clarifiers were undersized. Today, they pose problems in wet weather.” During these periods of peak load, heavily loaded inflows interfere with the functioning of the treatment cycle. To counter these problems, the capacity of the clarification process needs to be increased. However, it is not always easy to update a traditional clarifier, and it often has to be rebuilt. To save space, while increasing the capacity of the equipment, less bulky lamella clarifiers are used.
Degrémont’s Densadeg model is an example of the latest generation of equipment. The design incorporates both rapid flocculation with sludge recycling and slow flocculation to increase floc size. The Densadeg accepts a high settling speed (20 to 120 m/h), is unaffected by variations in speed or load, and removes 80 % of particles in suspension and 80 % of phosphorus.
Built-in sludge thickening achieves concentrations of 30 to 550 grammes, which can then be extracted and immediately sent for dewatering.
This is also the case with OTV’s Actiflo, which combines ballasted flocculation and lamella clarification. It is installed at plant inflow, just after fine screening and removal of coarse matter. Downstream, the treated water can be discharged into a river or sent for subsequent biological treatment. The sludge and micro-particles settling at the bottom of the clarifier are collected by a sludge collector or trough before being pumped to hydrocyclones for grading by size and to allow virtually all the micro-grit in the underflow to be recovered for recycling.
An advantage of these units is the reduced time that the effluent remains in the system. Furthermore, they can be quickly brought into service, and can therefore handle peak flows of stormwater.
Updating a water treatment works can go beyond simply managing excess flow. The regulations now require the treatment of nitrogen and phosphorus and the removal of microorganisms. Taking all these factors into account, fixed film growth reactors provide a way of treating the whole problem.
Using fixed film biomass growth
“It is important to be able to maintain a higher level of biological activity in the system,” states Pierre Gilles, who advocates the
Biolift technology to deal with these problems. The Biolift uses the technique of bacteria grown on a biological fluidised bed. Nitrifying bacteria adhere to the dense loose material (micro-grit). The circulation of water created by a column of air maintains the material in suspension.
This equipment is installed between the activated sludge basin and the clarifier. Nitrification takes place within the Biolift. At the exit, a recirculation loop returns the liquor mixture to the head of the activated sludge basin. The nitrates are broken down into gaseous nitrogen and oxygen, used for carbonaceous pollution degradation. The main circuit carries the liquor to the clarifier, where the activated sludge is separated from the treated water.
Water recirculation within the reactor allows self-cleaning of particles, thus eliminating excess biomass.
In contrast to conventional solutions, where the effluent remains for 10 to 24 hours, the Biolift reduces the contact time to 3 to 4 hours for denitrification and less than one hour for nitrification.
This compact unit is well adapted for use in the context of refurbishing a water treatment plant. In March 1998, OTV won the contract to equip the treatment works at Nancy Maxeville. Degrémont’s Biofor is in direct competition. This compact process is responsive to the needs of load shifts in tourist areas and industrial zones. It provides a flexible and economic solution for:
- - nitrification and denitrification,
- - elimination of iron and manganese,
- - elimination of phosphates and carbonaceous pollutants.
However, membranes could well be the solution for the future. The manufacturers are ready to meet the challenge: Degrémont, OTV, Orelis, Memcor, Tami Industries, etc. are all prospecting the residual water treatment market (see EIN No. 211, April 1998). While the investment costs are still considered to be too high, they are continuing to fall. If one takes into account the initial investment and operating and maintenance costs, the technology may well prove to be competitive.
