New method to optimise chemical precipitation in raw water and wastewater treatment plants
In chemical precipitation, Aluminium salts are often used. These are either monomeric Aluminium sulphate (ALG) or polyaluminium chloride (PAC). If the plants choose to use a PAC, the degree of polymerisation (basicity) must also be selected. After the choice is made, the only tool the operating crew possesses to achieve an acceptable treatment result is the dosage of Aluminium. By using Flocell’s new method, the dosage of coagulant and basicity can continuously be regulated in situ, which presents new possibilities to achieve better treatment results at a lower cost.
The following factors can negatively affect the conditions to achieve optimal treatment results:
Municipal wastewater treatment plant:
- Increased hydraulic load and degree of contamination.
- Unforeseen discharge from industries connected to the plant.
- Disruptions in the plant’s internal treatmentprocesses.
Internal industrial treatment plants:
- Disruptions in the plants’ internal treatment processes.
- Restart of the plants’ treatment process after long periods of production halts.
- Disruptions in production that lead to increased contamination.
- Change of raw materials in the production process.
Water treatment plant with a constant water production:
- Heavy rains that change the degree of contamination in raw water.
- Fluctuations in the temperature of the raw water and degree of contamination pertaining to seasonal changes.
- Unforeseendischarges of contaminants in the source of raw water.
- Long-term changes of the quality of raw water due to climate change.
It is not unusual that the choice of coagulant is based on results from laboratory-basedprecipitation trials, which seldom reflect variations in the operating conditions of treatment plants. In cases where the plants prioritise theseparation ofsuspended solids,a PAC is often selected, which then presents the problem of choosing an optimum basicity. The theoretical background is that a polymerised product has a higher charge than a monomeric product. Thus, the use of a coagulant with a high positive charge will lead to a more effective charge neutralisation since the particles in water usually are negatively charged. In cases where the plants prioritise the separation of dissolved organic matter and phosphor, the use of monomeric products such as ALG is often chosen over polymeric products. However, this choice also entails a compromise due to the fact that the hydraulic load as well as the type and degrees of contamination vary more often than not.
It is to overcome these challenges that Flocell introduces a new method that is patented in Sweden and patent-pending in China, US and Europe.This method involves regulating the basicity and coagulant dosagein situ by way of the continuous measurement of parameters pertaining to the contamination, such as phosphorus, turbidity, COD, TOC and colour. This requires an investment in measuring and regulating equipment, which is costly for smaller plants but is easily justified for medium and large plants. This investment also makes it easier for the operating personnel to quickly fix any errors and flaws in the plant.Furthermore, the recorded data from the treatment plants can facilitate the execution of proper environmental counter-measures by environmental enforcement agencies.
The process begins with a monomeric coagulant. A solution of the coagulantis introduced to hydroxide in the form of Calcium hydroxide or Magnesiumhydroxide slurry. The mixture is then allowed to react, whereby a polymerisation occurs. The basicityis continuously regulatedat the optimum level by controlling the molar ratio, i.e., the mole OH/Al. This method has been successfully tested on raw water on a pilot scale and on municipal wastewater on a full scale.
Raw water trials
The same raw water source has been used in the pilot plant and in Karlshamn Waterworks. The raw water is characterized by its high colour and high content of organic substances, as well as a relatively low turbidity. Karlshamn Waterworks uses a PAC with a basicity of about 45%.The average Aluminium dosage is 10 mg/l. The hydraulic load low. During the pilot tests, optimaltreatment results were obtained at a dosage of about 8 mg Al/l and a basicity of about 30%.
The trials in the pilot plant were conducted with ALG and the PAC that was used in Karlshamn Waterworks, as well as a polymeric product that was produced in situ.
In the trials, an ALG solution was used and its basicity was regulated in the range of 0-50% by the addition of Magnesiumhydroxide slurry. The trials were carried out from December 2014 to June 2015. The snow layer during that winter was very thin, and therefore did not affect the raw water when it melted. To examine how the treatment results were affected by increased turbidity,finely ground diatomaceous earth was added to the raw water. The result showed that approximately 8 mg Al / l and a basicity of about 20% yielded optimal treatment, i.e., a reduction of the previous basicity by 10%. This gives a good indication that a regulation of basicity and Al dosage provides better treatment results when precipitation conditions vary.
Karlshamn Waterworks, not unlike the majority of water treatment plants, uses the addition of lime to achieve the optimal pH for precipitation as well as to simultaneously increase the alkalinity of the water. If lime is used to increate the basicity, then the amount of lime needed for pH-adjustment will decrease equal to that required to increase the basicity. Hence, the operating costs will not be affected. It can, from the public health point-of-view, be an advantage to replace Calcium with Magnesium, as several scientific studies show that an increased content of Magnesium in drinking water reduces the occurrence of cardiovascular diseases.
The trials were conducted in full scale at Staffanstorp Municipal Waste Water Treatment Plant, which is designed for 320 m3 / h corresponding to 25,000 PE, but has a load today of only 15,000 PE. Here, biologically treated water is precipitated with ALG inthe average amount of 4 mg Al/l. The plantmet all discharge limits, when calculated as a monthly average, even if the limit for phosphorus exceeded the limit for short periods. In the trialsthe basicity of an ALG solution was regulated withMagnesiumhydroxide slurry. During the trial period, which lasted from February to June 2015, the plant was subjected to no significant amount of additional water from rainfall or snowmelt. The basicity was varied in the range of 0-45%, but no direct relationship could be detected between the basicity and total content of phosphorus in the outgoing treated water. The same was seen in regards to the nitrogen and COD content. However, an increased basicity yielded lower turbidity, which confirms the theory that an increased basicity creates better conditions for the separation of suspended solids. To better examine this relationship, two of the four sedimentation tanks were turned off after the activated sludge process and in the chemical precipitation stage. The purpose was to simulate an increased hydraulic load. By increasing the basicity during the trial from 0 to about 35%, the content of phosphorus was reduced from having exceeded the limit of 0.3 mg / l to <0.1 mg / l, despite that the Al dosage had been reduced from 4 to 3.5 mg / l.
In wastewater treatment plants, hydroxide is rarely used to regulate pH and theaddition of lime can therefore be expected to increase the cost of operation. An optimisation generally means thatthe Al dosage is kept as low as possible and that the basicity is used to meet the specified discharge limits. The reason for this is that Al is more expensive than hydroxide and that increasing the Al dosage increases sludge production and disposal costs. An addition of Calcium orMagnesium slurry can also have a positive effect on flocculation as the ions act as a co-coagulant. A better particle removal in the wastewater treatment process would probably also lead to reduceddischarge of micro-plastics. It is also likely that the bacterial levels in the wastewater would be reduced, which has a direct impact on the quality of bathing water in areas near the sewage treatment plant discharges.
Global warming will continue to increase the frequency of heavy rains in the future, which will cause problems in both raw water and wastewater treatment plants. Flocell’s new method minimises these problems and the negative environmental effects that global warming entails.