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Despite the complexity of the chemical reactions involved, there are some simple rules to guide effective effluent treatment
By Leslie Webb
Radical solution in effluent treatment
Large wastewater treatment plants are often considered to be the most environmentally sound in terms of emissions to the aquatic environment. But this is not necessarily true in relation to the plant's total environmental impact. A large wastewater treatment plant often means that the wastewater loads are high, almost certainly as a result of significant process losses and inefficient use of raw materials. If the wastewater has to be treated to a high standard before discharge, the quantity of treatment residues (better known as sludge) will also be considerable. This can lead to other consequences, depending on how the residues/sludge are dealt with. Even if the sludge is 100% beneficially re-used off-site, there will be an impact due to transportation.
A large wastewater treatment plant also means that hydraulic retention times will be on the high side and, in some process stages, anaerobic conditions can easily develop. In turn, this can cause operating difficulties and emissions to the atmosphere of undesirable gases such as hydrogen sulfide and hydrogen. An environmentally sound wastewater treatment plant involves more than just producing a discharge that complies with the regulated standard. It should also:
- minimize other inputs such as energy, chemicals and construction materials
- recover materials for re-use wherever possible
- avoid generating undesirable byproducts.
A specific issue arises when dealing with the non-biodegradable, but non-toxic, COD (chemical oxygen demand) that remains after biological treatment. Removal of this material is likely to require considerable inputs of other materials, notably energy and/or chemicals, to divert it from the water environment to either air or land.
Box 1 - Treatment of Coating Wastewater
Settle down or float up?
Sedimentation and flotation are the two most popular choices for solids/liquid separation, although the other candidate in the triumvirate - filtration - has been used at a few mills for primary wastewater treatment.
Compactness is not the first thing that comes to mind when describing sedimentation systems. Unfortunately, but not unsurprisingly, raw wastewater from pulping and papermaking is dominated by small particles with low natural settling velocities (0.5-1 m/hr). One way to speed this up and to use smaller tanks is chemical pre-treatment for coagulation and flocculation.
The UK manufacturer of aluminum compounds, Laporte Absorbents, has recently developed a new range of polyaluminum chlorides (PACs) made by a patented low energy process. These particular chlorides have a higher charge density than other PACs, which may have some advantages for papermakers as well as for wastewater treatment.
Coating wastewater is not difficult to treat, but it does require relatively high doses of inorganic salts for initial coagulation prior to settlement. If the mill is on the coast, simple mixing with saline waters can be adequate, but the residual salts do not help when recycling the clarified water. As Box 1 shows, alum is an effective coagulant. But the required dose can be drastically reduced by switching to PAC, particularly one with a high charge.
Mechanical tricks to speed up the sedimentation process also improve compactness and the best-known technique is the incorporation of lamella plates to increase the effective settling area. Although flotation is inherently more compact than sedimentation simply due to the high separation velocity of the air-buoyed particles, its size can be reduced even further with lamella plates.
KWI (formerly Krofta) has developed a dissolved air flotation unit of this type. The Megacell can operate at loadings of up to 30 m/hr. This compares to no more than about 8 m/hr for conventional flotation systems. The new design is based on special U-shaped elements in the tank that give an initial co-current flow pattern, followed by a counter-current phase for the removal of fine solids.
Megacells have already been delivered to several mills in Europe as pre-assembled units for rapid installation and startup. Lower operating costs are reported as a result of reduced wastewater recycling for air dissolution and lower chemical inputs for flocculation.
Tiny bugs but big plants
Although the actual bugs that do the hard work during biological treatment are individually very small (about the same size as filler particles in papermaking), the plants where they live tend to be very large. Sometimes, such plants can appear to be quite small, but this may be due to the plant being mainly hidden beneath the ground such as is found in the Deep Shaft process.
Biological processes can often benefit from having a large volume in terms of process stability, but the retention time of the active biomass (referred to by various names, but most commonly to as "sludge age") is more important than the liquid or hydraulic retention time. The sludge age must be greater than a particular period, which reflects the growth characteristics of the microbes in the treatment process. The sludge age parameter can run from as low as a few days for the mixture of aerobic micro-organisms in activated sludge plants to several weeks for the slower-growing micro-organisms in anaerobic treatment systems.
It is usually not economic to treat wastewater in 'once-through' plants (where the retention time of the liquor is the same as that of the biomass) because very large treatment tanks are needed in order to achieve the required sludge age. There are exceptions to this though, such as aerated lagoons, which are operated at hydraulic retention times of about five days. But this approach is not widely used unless there is plenty of land available. Concentrated wastes can be dealt with in this way if the high concentration is a result of some prior volume reduction, eg anaerobic digestion of sludge.
In conventional bio-treatment plants, the size of the main bio-reactor depends on the raw BOD (biochemical oxygen demand) load for treatment and on the applied BOD loading on the process that is considered acceptable to achieve the required effluent quality. For a given raw BOD load, treatment standard and operating conditions (temperature, pH, etc), it is the plant loading that determines the tank size and this depends principally on the tank’s biomass capacity.
In turn, the tank’s biomass capacity depends on the ability to supply the biomass with enough food for its requirements and the ability to separate the biomass from the liquor.
Box 2 - Different Types of Plant for Biomass Retention
In aerobic plants, the rate of oxygen supply can be improved by increasing its saturation concentration in water - either by using air at high pressure or pure oxygen at atmospheric pressure. There are many activated sludge plants treating wastewater from pulp and paper mills using pure oxygen and a smaller number using deep tanks or shafts.
The retention time of the liquor and biomass can be de-coupled by various means, giving rise to a number of different bio-treatment technologies in which the hydraulic retention time is much less than sludge age (Box 2).
In terms of simplicity and compactness, the most attractive approach is where the micro-organisms are kept in the reactor either through their natural characteristics or through mechanical means and no external separation is needed. The best example of this approach is the upflow anaerobic sludge blanket (UASB) reactor, where the micro-organisms grow naturally in a dense, granular form.
The latest application of this system is in the treatment of kraft evaporator condensates for methanol removal. The first plant installation took place last year, where Paques supplied an IC reactor to a Boise Cascade mill in Alabama.
The research that led to the UASB reactor started off with an anaerobic biofilter, but this approach has faded away with the success of the UASB concept. However, bio-filtration is still widely used in various guises for aerobic bio-treatment, most commonly in the form of the trickling or percolating filter, although this system has never been widely used in the pulp and paper sector.
In any type of biofilter, the retention time of the biomass is high because the micro-organisms are attached to, or immobilized on, the "filter" medium placed in the tank. But the term "filtration" is rather misleading as there is no real removal of particulate solids by a straining mechanism. This type of bioreactor is better described as a fixed film process and more accurately reflects the way that the biomass grows irrespective of the precise type of reactor. The traditional 50 mm stone media in aerobic biofilters are rarely used in new plants. Instead, modern units employ either some form of shaped plastic that has a high specific surface area and void volume, or small mineral particles with a high specific surface area, but lower void volume.
Box 3 - Ozone Treatment and Biofiltration of Mill Wastewaters
In the last 10 years or so, aerobic biofiltration has started to find some niches within wastewater treatment, notably at paper mills that generate rather weak effluents (BOD < 100 mg/l) after primary treatment. While the traditional trickling biofilters are open structures where aeration of the media takes place through natural ventilation, the systems recently installed at some paper mills for conventional secondary treatment are submerged reactors with mechanical aeration. As these reactors are similar to activated sludge plants, albeit without biomass recycling, they began to blur the distinction between these two types of biotreatment process. This has been further developed by the so-called moving bed bioreactors, which utilize attachment media, or biofilm carriers, in an aeration tank (PPI, June 1998, pp 39-43).
A similar process is the Floobed system from Valmet Flootek, which has been used to extend the treatment capacity at a large mill in Poland. The company produces unbleached kraft and neutral sulfite semichemical pulp along with packaging papers. This mill used to operate a conventional activated sludge process to achieve discharge limits of 50 mg/l BOD and 200 mg/l COD, but the company wanted to tighten these levels to 15 mg/l BOD and 150 mg/l COD from this year. This level of performance could have been achieved by a straightforward expansion of the activated sludge plant, but the mill chose to modify its mode of operation by incorporating small polythene carriers (specific surface area 380 m2/m3) in the first half of the aeration tank volume. This allowed that part of the system to operate at a higher biomass concentration and the sludge age increased as a result. Treatment efficiencies were consequently lifted from 50% to 90% COD removal and from 70% to 93% BOD removal. Despite the increased operating costs, the Floobed installation had a positive payback time of 1.7 years due to the savings in the discharge fees payable in Poland.
Chemical help
Activated sludge plants are the most popular way of removing BOD from many different types of wastewater. In its conventional guise, the process is best described as a suspended growth bioreactor as the active micro-organisms are not attached to any surfaces. As a result, the hydraulic and biomass retention times are the same on each pass through the aeration tank, so the required minimum sludge age is achieved by separation and recycling of the biomass.
The normal way of achieving this is via sedimentation, where the size of the tank depends on the flow rate and the biomass settlability. But there are problems at this stage of the process, mainly the ever-present issue of sludge bulking, which is usually due to the excessive growth of filamentous bacteria.
The most common way of living with bulking at paper mills is the deliberate introduction of something that is toxic, usually some form of chlorine, to selectively kill the filaments.
A somewhat different chemical approach has been pioneered by the talc supplier, Luzenac, based on the addition of large quantities of talc to the mixed liquor. Case studies of the effects from talc addition at paper mills indicate improved sludge settlability and plant performance, but at the expense of a high ash level (40%) in the recirculating sludge.
Any chemical treatment like this is a preventive measure and does not address the fundamental problem, which requires improved understanding of sludge microbiology and chemistry. The use of selector tanks that exploit the differences between filaments and non-filaments has proven beneficial to many mills. The size and mode of operation of these tanks varies widely, but a Danish recycled paper mill has benefited from using an anoxic selector for several years.
As wastewater from paper mills has little nitrogen present, this type of selector means that nitrate has to be added to provide an oxygen source available to non-filamentous bacteria, but not to most filamentous ones. Fortunately, the Danish mill had the right sort of filaments for this approach to work so that the sludge settled to over 2% consistency.
Filter or float
An obvious way around the bulking problem is not to use settlement for sludge separation. Flotation has the potential to consolidate activated sludge to over 2%, but requires a larger tank than normal to function in a sludge thickening as well as clarification mode. Data from an Italian mill using a KWI Sedifloat unit as the secondary clarifier reports a floated sludge consistency of 4.5% from an inlet biomass concentration up to 7.0 g/l.
Surface filtration using conventional fabrics could be used for biomass separation, but this has never taken off commercially for secondary clarification. The more advanced membrane filters have not proved a commercial success as a treatment in their own right, but they are being used in one of the newer variants of the activated sludge method.
For example, the membrane bioreactor (MBR) process has a number of commercial variants such as the Japanese Kubota process and the Canadian ZenoGem process. The membrane module in the latter process, ZenoWeed, is utilized in the only known MBR application in the paper industry at the small French board mill, Papeteries du Rhin. This Biosep plant was installed in 1999 by OTV to treat a wastewater flow of 900 m3/day containing daily loads of 3.6 ton COD and 1.35 ton BOD. The guaranteed treated wastewater quality is < 5 mg/l TSS, <15 mg/l BOD and <150 mg/l COD.
In MBR processes, not only does the membrane unit replace the conventional sedimentation tank used to separate the biomass solids from the liquor, but it can also be located within the main aeration tank, saving even more space. Due to the high biomass level in the mixed liquor (about 18 g/l compared to typically 3-6 g/l in a conventional plant), the loading on the full-scale plant (about 0.9 kg BOD/m3/day) gives a very high sludge age of about 50 days. This accounts for the low sludge production of about 0.15 kg/kg COD removed.
The economics of the MBR process versus more conventional activated sludge systems depend on the flow rate, which determines the size and cost of the membranes, and on the lifetime of the membranes. The first plants were installed some seven years ago and the original membranes are still in use. MBR systems also offer advantages over conventional activated sludge systems in terms of ease of water recycling back to the mill. This is mainly because the smaller size of these systems allows them to be positioned close to the mill. In terms of water quality, they can remove not only particulate solids, but also higher molecular weight dissolved organics that contribute to residual wastewater COD.
COD conversion to BOD
As mentioned at the start of this article, the residual COD that remains after biotreatment can be removed. But this is not cheap and involves significant inputs of energy and/or chemicals. The use of membranes in their own right as a tertiary stage after biotreatment could accomplish this and there are a few full-scale examples in operation (PPI, April 1999, pp 29-32).
Wastewater evaporation can remove most dissolved solids apart from volatile substances. But commercial exploitation of this well-known technology stalled after a few plants were installed a couple of years ago, although the hold-up may be only temporary.
One of the problems with both membranes and evaporation is what to do with the concentrate left behind. With this in mind, the Finnish company, Conox, has developed a pressurized thermal oxidizer. It uses pure oxygen rather than air for combustion at high temperatures (1,200-1,700°C) and has been applied on a pilot scale at a Swedish pulp bleaching plant and a straw pulp mill in China.
A technology that has been around for some time, but not applied on a mill scale until last year, is wastewater treatment with strong oxidizing chemicals. Chlorine, chlorine dioxide and peroxides have been used for many years within wastewater treatment plants, but more to deal with operating problems such as odors than as a treatment in their own right. Most of the technical development on wastewater ozonation after biotreatment has been conducted in Germany on wastewater from integrated mechanical pulp mills and recycled paper mills. The treatment involves partial oxidation of the organic molecules to enhance their biodegradability followed by a further small bio-treatment stage, typically a biofilter, to lower both the final BOD and COD.
An example of the results that can be achieved is illustrated in Box 3 for wastewater from two mills over a two-stage process. The amount of ozone consumed increases with the required COD removal, going from about 1 kg ozone/kg COD removed when the COD removal is around 50%, to about 2 kg ozone/kg COD removed when the COD removal is 80-90%.
The latter efficiencies were achieved in pilot scale work at the SCA Laakirchen mill in Austria, where improved COD removal may be required if a large mill expansion goes ahead. Part of this improvement could be achieved by incorporating a moving bed bioreactor before the existing activated sludge plant and the rest by post-ozonation and biofiltration with a target of 94% total COD removal.
However, this would not be the first full-scale wastewater ozonation plant. Wedeco installed such a system at the Lang mill in Ettringen, Germany, last year. This mill was in a similar position of rising BOD/COD loads due to mill expansion, but with no possibility of discharging higher loads to the receiving watercourse. Although the discharge BOD concentration was low (<10 mg/l), the COD was about 400 mg/l giving a specific hard COD load (3.0-3.5 kg/ton of paper) that is fairly typical of this type of mill.
As a result, the BOD to COD ratio of <0.025 is very low. Indeed, it is much lower than most clarified raw wastewater with a ratio of 0.4-0.5, but exactly what would be expected after biotreatment. Pilot-scale trials gave COD reductions in the combined ozonation/biofiltration step of 20-40% at ozone doses of 0.2-0.4 kg/COD applied. Early results from the full-scale unit showed a COD removal of nearly 50% at an ozone dose of 0.6 kg/COD applied, taking the final COD comfortably below the required standard of 300 mg/l. The retention time in the ozonation plant is one hour to allow for complete consumption of the ozone before the wastewater passes to the upflow oxygenated biofilter (volume 170 m3) for removal of the ozone-generated BOD.
Chemicals instead of bugs
As can be seen at the Lang mill, the conventional wisdom is that biodegradable organics in the raw wastewater are removed most cost-effectively by biological treatment. But the benefits of pre-ozonation have been researched in at least two countries - Canada and Finland. For wastewater from mechanical pulping, ozonation appears to attack the wood-derived toxic components preferentially over the rest of the organics. But as the ozone still reacts with all the organics to some degree, the doses remain high and could not be justified unless bio-treatment proved ineffective.
Ozone in the presence of ultraviolet light forms an even stronger oxidizing species - the hydroxyl radical. This can also be generated by a number of other methods such as hydrogen peroxide plus iron salts (Fenton's reagent). The topic has been the subject of sporadic research for application in the paper industry. The latest work is being carried out in South America on the removal of organochlorine compounds from ECF (elemental chlorine-free) bleaching wastewater, something that is not easily controllable during normal biotreatment. However, a small UK mill has recently grabbed hold of this concept and replaced its activated sludge plant with an electrochemical system that uses a Fenton-type reaction. Whether this proves the universal remedy for wastewater treatment remains to be seen, but it is a very radical approach in more ways than one.
Leslie Webb directs the activities of Envirocell. He can be contacted on tel/ fax at +44 1372 276599, leswebb@envirocell.co.uk or www.envirocell.co.uk.
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