Experience from early treatment system installations provides knowledge for better methods of operation and control

Improved Control Makes Activated Sludge Treatment More Viable

Today, similar installations are being made at mills in developing countries such as Indonesia. Traditionally, the activated sludge method has been widely used in both Europe and Japan, where it has proven effective in regards to removal of organic matter in connection with other methods. In the paper
industry, the commonly perceived drawback of acti-
vated sludge systems-namely the control of nutrients (phosphorus and nitrogen)-can actually be handled so efficiently that the effluent to the receiving stream contains lower concentrations of the nutrients than what is typically found in the influent.

The common opinion is that wastewater treatment is an unnecessary activity and that effluent should be reduced to an absolute minimum through different process modifications. In reality, effective treatment of wastewater has "bought" mills time to develop techno-economical alternatives for "closed mill" process modifications. In spite of all the time invested in this area, considerably more time seems to be needed until effluent free mills can be realized.

The planning and construction of activated sludge plants has not been without problems either, especially since there are some economic restrictions within which mills need to operate. A good example of this is all the activated sludge plants that were built during a short time period in Finland in the 1980s. The biggest mistakes were corrected by making modifications and additions to the original plants. During the past decade almost every single plant built has been based on straight plug flow plants, equipped with selectors.

COMMON OPERATIONAL PROBLEMS. The following will review operating experiences from different pulp and paper mill activated sludge treatment facilities and what has been done to minimize problems and develop control strategies for these plants. Experiences include facilities in Finland, Canada, Germany, and France.

In Finland the most common causes of operational problems in plants built during the 1980s were high total suspended solids (TSS) in the influent to the plant, insufficient aeration capacity in the aeration basins, and insufficient capacity to handle sludge de-watering. Furthermore, some of the plants were built with aeration basins too small, a design problem in that sludge swelling led to the loss of sludge from the secondary sedimentation basins. The same problems were experienced if the secondary sedimentation tanks were too small.

Even though these problems have been widely communicated, some of them took a long time to correct, and, actually, some continue to be ignored. However, there has been a clear improvement in the recently-designed and built plants where such problems have basically been avoided. This indicates that the exchange of experiences between mills and corporations have resulted in system improvements.

In the meantime, the production capacity of most mills has been increased substantially, and often these increases have been supported by a parallel increase in wastewater treatment capacity. In the Nordic countries, where climatic conditions vary substantially from summer to winter, a considerable increase (20% to 30%) in wastewater loads during the winter season has been experienced. These increased loads often also result in operational problems, with the most common being "bulking sludge" leading to some sludge escaping from secondary sedimentation.

CAUSES OF OPERATING PROBLEMS. The most common causes of operational upsets are insufficient process control, permanently or temporarily malfunctioning equipment, and unusual changes in the characteristics of the influent.

The first unit process in an activated sludge treatment plant is sorption. Suspended solids, colloidal, and dissolved material are all attached to the bio sludge. The return sludge to the primary sedimentation settles well, and its growth is encouraged. It is important to have enough return sludge in relation to the incoming load of material.

The common measure is lb COD/lb of return sludge, measured in the same time units. This is often called the "floc load." The sorption is most effective in selector compartments or in plug flow reactors. A very common mistake is to use too high a floc load in the treatment of highly concentrated pulp and paper industry wastewaters. The organic matter will be attached to the sludge through sorption later in the process, and this leads to bulking sludge.

Further, it is notable that pulp and paper industry wastewaters are low in phosphorus and/or nitrogen. Thus, it is still common to see errors in the dosage of nutrients, even though it has improved through the years. The shortage of nutrients will have an impact on the biodegradation process, which follows after the sorption.

In a plug flow reactor, the oxygen uptake is usually at its maximum 1.5 to 3 hours after the sorption and at its lowest around 6 to 8 hours after the sorption. This requires a proper distribution of the aeration capacity as well as good agitation. Usually approximately 50% of the aeration capacity should be allocated to the first third of the basin. It is very common though that this has been neglected for a variety of reasons. Sometimes the water is introduced step wise to the aeration basin instead, which leads to a different oxygen demand in the aeration. This, however, often results in bulking sludge.

Oxygen demand can be increased to exceed capacity by unnecessarily increasing the sludge age, which means increasing the sludge volume. By increasing the sludge age, the production of bio-sludge is decreased, the removal efficiency is increased, and the need for nutrient addition is decreased. Thus it is justified to increase the sludge age, but it has to be done within the limits of the aeration capacity.

Some other, less common, operational disturbances have included errors in pH control, high concentration of fatty and rosin acids, and lack of some essential metals.

Of the chemicals used in the production of pulp and paper, it is primarily peroxide and SO2 water from the bleach plant that sometimes has caused operational problems in activated sludge plants. In some instances, where an aerated lagoon and a bio-filter have been operating in series with an activated sludge plant, it has been found that the activated sludge plant has had unusually low efficiency. The primary problem has been high suspended solids in the effluent. The likely cause is a high concentration of microbes that do not settle well.

It has been found that the sorption of iron, zinc, manganese, and potassium has been very effective in activated sludge plants. Further, there is clear evidence that adding iron has improved the operation of the plants.

From an operational point of view, the most critical parts of the activated sludge plant are primary sedimentation, the aerators and their location, assurance of power to the compressors, spare compressors, and sludge dewatering. Insufficient or malfunctioning instruments often also lead to bad control; very common are unreliable flow and oxygen
measurements.

Earlier in their implementation, it was also common in Finland to have sizing errors of both aeration and sedimentation tanks. Further, the sludge removal systems in secondary sedimentation have caused operational problems. The return sludge comes out at low consistency, which leads to too high a floc load.

Some of the most common problems, their causes, and suggested solutions are illustrated in Table 1.

Table 1. Common problems of activated sludge treatment plants along with typical causes and their solutions.

• Better removal of suspended solids from the influent
• More uniform hydraulic loading
• Reducing the amounts of toxic substances entering the plant.

From a process control point of view the biggest problems in a plug flow plant are the badly managed sorption process, the uncontrolled addition of nutrients, and lack of aeration control. For control purposes, the sorption and oxygen transfer characteristics can easily be determined in the laboratory.

For nutrient addition (N = nitrogen and P = phosphorous), three different approaches are used:

• Addition of nutrients in relation to BOD load-i.e. BOD5:N:P = 100:4:1
• Addition of nitrogen and phosphorus so that the effluent contains 0.1 to 0.2 mg P/liter dissolved phosphorus and 1.5 to 2 mg N/liter (or 0.1 to 0.2 mg NH4/liter)
• Monitor the nutrient balance-that is, incoming and outgoing amounts of nutrients should be the same over a longer time period. Also, make sure that the effluent has the proper concentrations of dissolved nitrogen and phosphorous.

Of the above methods, the first one (ratio control) often leads to wrong dosage, because no consideration is given to the varying incoming amounts. Also, this does not consider the nutrient circulation in the activated sludge plant. The method can be enhanced by increasing the monitoring frequency for the nutrient concentrations.

The method of monitoring the dissolved nutrients in the effluent will, most of the time, provide good results. Errors occur primarily during upsets when the dissolved nutrients increase. This would be the case if the oxygen level was temporarily low. The monitoring of nutrients needs to be done by the use of reliable, continuous measurements.

By far, the most reliable method is in using nutrient balances and determining the dissolved nutrients in the effluent. In this way, low outgoing effluent nutrient levels can be achieved-for example, 0.2 to 0.4 mg P tot/liter and 2 to 4 mg N tot/liter. The cost for nutrient addition is also considerably decreased.

The use of a return number as a control parameter has also proven to be very useful in optimizing the operation of the secondary sedimentation. Thus, it can be ensured that the sludge is returned enough times (25 to 30) to secondary sedimentation. This will maintain a large number of well-settling bacteria in the return sludge.

DR. PERTTI HYNNINEN is with EnviroData Oy, Helsinki, Finland, and LARS C. INGMAN is president, Conmark Systems Inc., Norcross, Ga.