BIOLOGICAL TREATMENT
Removal of Ammonium and Nitrates
The reduction of Ammonium and Nitrates is accomplished by a combined process called nitrification / denitrification (NH4 -> NO2 -> NO3 -> N2). Apart from temperature and pH value, the sludge age and concentration of degradable organic components is of importance. In order to use the full denitrification capacity of the treatment plant, a continuous measurement of the nitrate concentration at the outlet of the pre-located denitrification step is useful. The recirculation of the wastewater into the denitrification tank is controlled with this measurement very efficiently. A low nitrate value, measured at that location, means a most efficient degree of recirculation. This means cost savings. The measurement can be done using optical sensors Stamosens CNM 750/CNS 70.
Removal of Phosphates
A high concentration of phosphates in the effluent of a water treatment plant leads to significant algae growth in rivers and lakes. Because of this, phosphates must be eliminated during the water treatment process, by dosing chemical precipitants such as iron chloride into the waste water. Often, the contamination of the waste water with phosphates is not constant; therefore flow proportional dosing is not efficient. Continuous measurement of the phosphate concentration ensures optimized dosing of chemicals and helps to save money. Controlled removal of phosphates leads to reduced sludge development which means additional cost reduction in sludge treatment as well. Chemical analyzers such as CA 71 pH, including the sample preparation, measure the correct values needed for efficient dosing of the chemicals online.
SLUDGE MANAGEMENT
Process efficiency
In waste water treatment plants, large quantities of sludge must be handled. Sludge has to be removed in the primary clarifier, recirculated as activated sludge in the biology and separated from the treated water in the second clarifier. Most countries have very strict regulations regarding the maximum load of sludge particles in the effluent of the treatment plant. Getting rid of the sludge separated from the water is an important cost factor and will become more costly in the future.
Primary sedimentation tank
In order to ensure efficient water treatment, the primary sludge has to be removed. The task is to control a pump or slide valve. Most essential is to make sure that the sludge concentration is at least 1.5 to 2% DS (dry solids). A lower concentration will create tremendous costs in later stages of sludge treatment (e.g. sludge conditioning and dewatering). The CUS 41, an optical sensor, is most suitable for measuring the solids concentration directly in the sludge pipeline and can be used easily to switch off the pump at too low concentrations. For sludge level measurement in the primary clarifier, an ultrasonic device such as CUM 750/CUS 70 is highly recommended, since it does not come into direct contact with the difficult product and it has no mechanical parts that can be blocked.
Second sedimentation tank
Sludge collected in the second sedimentation tank is different from that in the primary clarifier. Pump control and concentration monitoring is important. Often there is additional functionality needed: the sedimentation behavior, the sludge quality and also the sludge age must be known.
This information can be determined by a sludge concentration profile. Such a profile can be determined with a CUC 101 unit, an optical sensor which measures the different sludge concentrations between the bottom of the sedimentation tank and the water surface.
Sludge digestion
The production of biogas in the digester leads to a reasonable cost reduction. Up to 60% of the electrical energy needed in the sludge treatment plant can be produced by biogas if the sludge is well conditioned. Sludge concentration, temperature and pH value have to be kept stable. The beginning of any sludge treatment is sludge thickening. This is carried out in static sludge thickeners, centrifuges or flotation cells. Primary sludge and excess sludge is mixed with a coagulation agent. By adding chemical additives, the pH value is maintained to ensure that the methane bacteria work most efficiently. Both parameters, pH and sludge concentration are measured online in the sludge digestion pipeline to the digester. The sludge is kept in the digester approximately 28 days at a temperature between 104° (40°C) and 122°F (50°C) according to the process used. During this time the sludge is permanently circulated. Under less ideal conditions the sludge in the digester has a tendency to develop foam. The worst case is if the security valves open and the Foam contaminates the installations on top of the digester. To avoid this situation, often there is a second level measurement installed at the top, in addition to the standard hydrostatic level measurement which is installed at the bottom of the digester. The Micropilot FMR 230 can detect the foam in the digester so that de-foaming steps can be taken in time and biogas production losses can be avoided.
Sludge dewatering
Digested sludge contains about 95% water. Before the sludge can be deposited in a landfill or burned, it must be de-watered in thickeners, belt filter presses, or centrifuges. All thickening processes usually require flocculation agent dosage, which is controlled by a volumetric flow measurement, and a sludge concentration measurement. Exact dosing of polymers is important because a dosage that is too high increases costs for the chemical agent and decreases the efficiency of the dewatering process. In addition to the cost savings for chemicals, the dewatering reduces the sludge volume so that disposition costs are reduced as well.
