Recirculation of sludge-water in the water treatment process – a pilot study

The subject of a sufficient supply of water resources and, along with it, the increasing price of water are coming to the forefront of interest for both professionals and the general public. This is particularly relevant for water works companies, due to the fees for water intake for treatment, and the release of waste water, increasing over the years. This trend is likely to continue. These and other factors lead to the increase of water costs, which the general public resists. This puts considerable pressure on water works companies to increase the economy of their operations.

One solution might be the effort to decrease the volume of waste water produced in treatment plants. Waste water basically forms throughout the entire technological process (Crittenden et al. 2005). A large portion in particular is produced during the flushing of filters and removal of sludge from sediment basins (Kyncl 2007). This has already triggered extensive reconstruction work in some treatment plants and has led to the implementation of filters with better operational parameters. Another method, also suitable for water treatment facilities that do not plan extensive reconstruction, is an effort to re-use backwashing water; returning as large a volume as possible for treatment (Arora et al. 2001; Edzwald et al. 2001).

This measure may only be implemented upon evaluation of the risk factors (Tobiason et al. 2003; Gottfried & Walsh 2008; Zhou et al. 2013) that might prevent the return of backwashing water back into the treatment process. The properties of the raw and backwashing water must be considered (Gottfried et al. 2008), as well as the water produced by their blending. Another consideration concerns the treatment plant's ability to process water with parameters differing from the original condition. The appropriate amount of backwashing water that can be safely returned to the treatment process can be established based on both water analysis and an evaluation of the entire operation.

Within this research, possibilities for re-use of sludge-water within water treatment were verified on a specific water treatment in locations where previous measures were already implemented. The purpose of this research is the commencement of operations with repeated maximum use of backwashing water. The results provide the basis for an evaluation of whether returning a larger amount of backwashing water back into the water treatment process is possible without negatively affecting the treatment process, as well as a further evaluation of the financial savings that such a change might yield.

The subject of the study is the possibility to increase the use of backwashing water in water treatment plants by returning it to the raw water inlet. As the operational and technological conditions are (and can be) significantly different in various treatment plants, the described study refers to a particular water treatment facility.

Aside from drinking water, the quality of which is clearly defined, the following text also differentiates water, or more specifically sludge, traveling through various stages of the treatment process, as follows:

• Raw water – water brought in from a dammed reservoir intended for further treatment prior to producing drinking water;

• Backwashing water – treated water corresponding to drinking water in terms of quality and intended for the flushing of filters;

• Fresh sludge-water – backwashing water used for flushing filters, without treatment;

• Cleansed sludge-water – water obtained by the thickening of fresh sludge-water. This is the clarified part of the volume of water flowing from thickening basins;

• Returnable sludge-water – a portion of the sludge-water that is recirculated back into the treatment process;

• Sludge – thickened sludge-water;

• Mixed water – mixture of raw water and its respective quantity of cleansed sludge-water.

The study was carried out in a specific water treatment plant with a surface water source (dammed reservoir) and the following composition of the technological system. Raw water (the quality of which being shown in the Figure 1) is piped from the reservoir into the inlet facility. The next step is pre-oxidation with ozone and dosage by a coagulant (aluminum sulfate) that causes coagulation and flocculation in the treated water. Other supplemental chemicals can also be added, such as an ancillary flocculant or potassium permanganate.

The water is then guided through silica-sand filters, followed by ozone disinfection and pumping to the second separation level – filters with granulated active coal. The treated water is secured with chlorine dioxide and chlorine. Its pH is further adjusted, using calcium water. Drinking water is then accumulated in a water tower.

Backwashing water is obtained from the backwashing water basin, to which water is pumped from both the output accumulation and the segment immediately behind the sand filters. Backwashing water is used for cleaning both filtration levels and fresh backwashing water is then further treated by thickening in the sludge management process of sedimentation. The sludge obtained is removed from the facility and sent for further processing. The cleansed backwashing water is partly released into the gutter, where it is possible to bring it back to the beginning of the treatment process – to the inlet facility, where it is mixed with raw water.

Since 2013, in an effort to increase the economy of the operation, sludge-water has been mixed in with raw water at a ratio of 5:95.

It was determined that the evaluation of the intent to increase the ratio of the sludge-water to raw water would be executed in two phases. This study exclusively covers the method and results of the first verification stage. The goal of the research was the verification of whether this ratio could be increased to 10:90.

The values of selected indicators of water quality in relation to the changes in ratio of the sludge and raw water were monitored for the purpose of verifying this option. The following indicators were selected after consultation with the treatment facility operator:

• turbidity;

• color;

• chemical oxygen demand (COD-Mn);

• iron concentration;

• manganese concentration;

• aluminum concentration.

Monitoring of aluminum concentration was selected due to the use of aluminum sulfate as a coagulant, which considerably increases the concentration of aluminum in backwashing water.

The study used the results of analyses of raw, treated and backwashing water samples, particularly those collected in 2015, as the evaluation was performed at the end of that year. Further, supplementary analyses of the above indicators were performed using standard laboratory procedures.

As apparent from the results of the monitoring analyses shown in Table 1, over the course of the monitored year the given water treatment facility produced drinking water that did not come close to the valid limit values for drinking water in any of the parameters, despite the fact that 5% of returnable backwashing water was added to the process.

An analysis of values identified in the monitoring of raw, treated and cleansed sludge-water was performed for the above selected indicators. Using a blending equation, the concentrations reached in the mixed water that contained 5% or 10% (of volume) of cleansed backwashing water were calculated for iron, aluminum and manganese.

In the case of the COD-Mn indicator in the mixed water, its values were established through laboratory experiments as described below.

As is apparent from the turbidity values shown (Figure 2), the highest values and largest variances in values were reached in the case of cleansed sludge-water. Yet despite this factor, approximately half of the turbidity values of this water were below the drinking water limit (5 NTU). Therefore, it was possible to estimate that the mixed water with a 10% content of cleansed backwashing water would have higher turbidity values. However, this fact would not significantly affect the treatment process. More specific values could be obtained through laboratory experiments. The increased turbidity in cleansed backwashing water may point to the presence of undesirable microorganisms.

An analysis of color values found that the values of both raw and cleansed sludge-water exceeded the limit for drinking water throughout the year. In the case of raw water, the highest measured color value was 69.0 mg Pt·l⁻¹ and in the case of mixed sludge-water it was 144.0 mg Pt·l⁻¹.

In the case of COD-Mn, the situation was almost identical to that of color. Both the values of raw and cleansed sludge-water were higher than those limits valid for drinking water (3 mg·l⁻¹) year around. By adding 10% of cleansed sludge-water into raw water, a worsened value of this indicator could be expected.

Given that the COD-Mn in mixed water could not be established by calculation, a laboratory stipulation was required to identify how mixing affects the final COD-Mn value. For this reason, COD-Mn was established using four sets of samples:

• cleansed sludge-water and raw water in the ratio of 5:95 (a 5% mixed water);

• cleansed sludge-water and raw water in the ratio 10:90 (a 10% mixed water);

• raw water;

• cleansed sludge-water.

The established COD-Mn values are apparent from Table 2 (showing only a selection of values). As is apparent from the measured values, despite the fact that the cleansed backwashing water showed significantly higher COD-Mn values, in both cases of mixed water this parameter increased only slightly.

The average value of COD-Mn of raw water was established at 4.76 mg·l⁻¹. Cleansed sludge-water had almost double the average COD-Mn value, specifically, 10.02 mg·l⁻¹. Once the cleansed sludge-water was mixed with raw water in the ratio of 95:5, the average COD-Mn of the mixed water increased to 5.27 mg·l⁻¹, which is approximately 11% higher than the COD-Mn of raw water. In the case of raw water being mixed with the cleansed sludge-water in the ratio of 90:10, the average COD-Mn value increased to 5.58 mg·l⁻¹, which is approximately 17% higher than raw water.

The aluminum concentration in the cleansed sludge-water was significantly increased, including values up to 2.88 mg·l⁻¹. The origin of these increased values is related to the dosages of aluminum sulfate coagulant (Edzwald 1993). Due to the quality of raw water during certain periods of the year (June-November), additional flocculant must be used because, without it, clumping is insufficient ahead of the inlet to the filtration. After the dose of flocculant, it was possible to remedy this undesirable condition and stabilize the aluminum values in the sludge-water at lower values. In springtime, aluminum concentrations in the cleansed sludge-water reached the mentioned 2.88 mg·l⁻¹. However, upon dosages of additional flocculant, the concentration decreased to an average value of 1.00 mg·l⁻¹.

As is apparent from both laboratory monitoring and calculations, the addition of cleansed sludge-water to raw water brings with it a high likelihood of increased concentrations of aluminum in the mixed water. However, despite this, the mixed water only rarely contains a concentration higher than the value limit for drinking water. Nonetheless, caution is required because the treatment process may be negatively affected by increased aluminum concentrations.

In approximately half of the cases, concentrations of iron and manganese in the cleansed sludge-water (see Figure 3) were below the limits stipulated for drinking water. Additionally, these values were considerably lower than the values monitored in raw water. This was apparently caused by very effective separation in raw water treatment, as well as during the thickening of sludge-water through sedimentation in the sludge management stage. The cleansed backwashing water would somewhat dilute raw water in terms of the concentrations of both these metals. Therefore, an increased ratio of returnable water from 5% to 10% should cause no operational problems.

During the first phase of the research, the following indicators were monitored in raw, treated and sludge-water: turbidity, color and chemical oxygen demand, as well as concentrations of aluminum, iron and manganese. Based on the analyses performed, as well as operational observations, we conclude that the increase of ratio of returnable backwashing water from 5 to 10% in raw water should not negatively affect the water treatment process.

In fact, in regard to the concentration of iron and manganese, a slight decrease of concentrations of these metals in raw water is expected due to dilution. In the case of COD-Mn values, color and turbidity, certain increases may be expected, but not to a degree that would significantly affect the treatability of the water.

The greatest emphasis during control should be applied to concentrations of aluminum, because its concentrations in the returnable water are significantly increased. With the increased ratio of the returnable water, this increase would reflect in the water intended for treatment.

Mixtures with 5% and 10% content of backwashing water were prepared to provide an example of how the increased amount of returnable backwashing water reflects in the COD-Mn indicator and this indicator was established in the mixed water. As is apparent from the measured values, despite the cleansed sludge-water showing significantly higher COD-Mn, in both types of mixed waters the increase of this parameter was only slight.

A graphic comparison of the individual indicators shows that the course of the values is very similar, although a risk period might occur should all monitored parameters be worsened in the raw water.

The conclusion of the first phase indicates that increased amounts of recirculated backwashing water for treatment should be possible. However, given that these were only theoretical results, a recommendation was issued to run periodic tests for a certain time to monitor the coagulation of mixed water with the 10% content of backwashing water. It was further recommended to monitor selected microbiological indicators (Cornwell & Macphee 2001) that were not considered in the initial phase, but may have an effect on the result of its intent.

Based on the results of the first theoretical stage that indicated that the implementation of the intent should be possible, the operator of the treatment facility decided to implement a higher volume of sludge-water in a ratio of 6% to raw water. This step alone brought considerable economic savings.

As is apparent from Table 3, while in the first half of 2016 the ratio of sludge-water in the volume of raw water was 4.6% and the ratio of unused cleansed sludge-water was 59%, during the second half of the year and upon increasing the use of sludge-water by 1% in the ratio toward raw water, there was a decrease in volume of unused sludge-water to 43.4%.

The study further indicates that, thanks to this change, on average during 2016, 50% of the original amount of backwashing water was released into the gutter. Therefore, this represented a significant increase of operational economy, without changing or otherwise disrupting the treatment process.

The analysis only presents the first phase, with the purpose of verifying whether it is possible to increase the ratio of returnable sludge-water in raw water. The following phase of research, which includes a broad series of coagulation tests, much more reliably proves changes of raw water quality due to the addition of sludge-water. This particular water treatment facility situation is complicated, because the facility operated within an interrupted schedule and so the quality of raw water from the surface source can be rather variable. For this reason, a sufficient number of experiments are planned to reliably prove the impact of mixing water as it relates to the quality of drinking water.

The quality of cleansed sludge-water will also be monitored during the next experimental stage, as the timing of withholding of backwashing water within sludge management differs considerably. During the operation of the treatment plant, this could be hours and if the treatment plant is not in use, backwashing water could be subject to sedimentation over several days. This might well result in differences in the cleansed sludge-water and these should also be evaluated in the next step.

Further, the following phase should also monitor selected microbiological indicators that were not considered in the first phase.

This article was drawn up within project No. LO1408 “AdMaS UP - Advanced Building Materials, Structures and Technologies” supported by the Ministry of Education, Youth and Sports as part of the targeted support programme “National Programme for Sustainability I” and under Project No. FAST-S-17-4643 supported by Brno University of Technology, special thanks to the operator Brněnské vodárny a kanalizace, a.s.