What you should know: Everything about wastewater treatment and small sewage treatment plants (5)

After important basic knowledge about wastewater treatment with small sewage treatment plants in parts 1 and 2 of our knowledge series and the explanation of common cleaning processes and plant types in parts 3 and 4, things are now getting really exciting again with the most advanced systems and processes. Especially in times of climate change, what counts in wastewater treatment is performance, economy and efficiency - after all, water protection is climate protection. Contemporary methods of wastewater treatment are more popular than ever...

Membrane Bioreactors (#MBR)

#Membrane bioreactors are operated with a free-floating #biomass. The microorganisms combine to form #sludge flakes, which can have different sizes depending on the #aeration process. The required aeration is generated with a compressor and fed into the #bioreactor via #membrane aerators. A #secondary clarifier is not required, since the water is drawn off from the bioreactor via #membrane filters.

The systems are approved where #wastewater has to be disinfected or #disinfection is required: discharge into bathing lakes or into #water protection areas are some applications. With #wastewater treatment, the #effluent classes C (#carbon decomposition), N (#nitrification) and H (#hygienization) can be achieved.

Special system features

Due to their #biomass structure, membrane bioreactors have a very good utilization of the reactor volume in relation to all other systems - the volume of the bioreactor is relatively small compared to other systems. Short-term load surges, which can occur especially in small sewage treatment plants up to 8 inhabitants, can therefore lead to a temporary deterioration in the cleaning performance. The ventilation must be designed in such a way that both the oxygen demand of the #microorganisms is covered and permanent #sludge deposits are prevented. The ventilation must also be designed in such a way that the air flow can keep the filter surfaces clean. In contrast to #biofilm systems, the multiplication rate of the microorganisms is much higher, so that the biomass can adapt more quickly to different dirt loads. Several filter plates are combined into blocks. The filter material consists of a plastic-coated carrier material with very fine pores, which are also able to retain harmful bacteria. The #water to be drained is usually sucked through the filter by pumps. The filter must be cleaned with chemicals at least once a year by a maintenance service.

Based on previous experience, the filter blocks must be completely replaced after 5 years. If the filter is intact, #turbidity is also removed from the wastewater. If filter plates are used instead of ceramic modules, the costs for cleaning them are lower.

What should be given special attention?
#Sludge drift from the #primary treatment must be prevented, as this can clog the spaces between the filter plates. Mechanical damage prevents the sterility of the draining water. Certain substances in the waste water, e.g. fats, can lead to early clogging of the filter material, so that cost-intensive cleaning at shorter intervals can become necessary. So that an increasing clogging of the membrane is detected at an early stage, a corresponding measuring device with an alarm that measures the negative pressure in the suction system is actually a matter of course. Damage to the filter material can also be detected by this measuring device due to a pressure drop in the suction system. A decrease in the air performance of the system or longer switching off of the ventilation leads to the #membrane gaps becoming blocked. The #membrane plates can then usually no longer be cleaned and must therefore be renewed.

What are the controls like?
If the water level in the bioreactor is too high, this could be a sign that the filter material is clogged or an above-average amount of wastewater has flowed into the #sewage treatment plant. In such cases, a backflow into the house is prevented by an emergency overflow, which is usually available. However, valuable #activated sludge or untreated wastewater from the preliminary treatment gets into the outflow, which can result in consequential damage in the form of #environmental pollution. If the water level is too high, the maintenance service must be informed.

Activated sludge treatment plants in back-up operation (#SBR)?

Diese Anlagen werden mit einer freischwebenden Biomasse betrieben. Die Mikroorganismen schließen sich zu Schlammflocken zusammen, die je nach Belüftungsverfahren unterschiedliche Größen haben können. Im Gegensatz zum #Biofilm kann der Fachmann über Farbe, Form, Geruch, Menge und #Absetzverhalten der Schlammflocken den Zustand der Anlage mit bloßem Auge beurteilen. Die erforderliche Belüftung wird mit einem Luftverdichter erzeugt und über gelochte Membranbelüfter in den Bioreaktor geleitet. Weitere Belüftungsmöglichkeiten bieten Tauchmotor-, Injektor- und Oberflächenbelüfter. Je nach Anlagentyp sind eine oder zwei Pumpen für den Wassertransport erforderlich. Einige Hersteller verwenden zusätzlich noch Magnetventile. Bei dieser Art der Abwasserreinigung können die Ablaufklassen C (Kohlenstoffabbau), N (Nitrifikation) und D (#Denitrifikation) erreicht werden.

Special system features
Due to their biomass structure, SBR sewage treatment plants have a very good utilization of the reactor volume compared to biofilm plants. Due to the prescribed dimensioning of the bioreactor in small sewage treatment plants, these plants can process at least twice the dirt load for which they were designed. However, the ventilation and the possible additional hydraulic load must be adapted to these conditions. The aeration must be designed in such a way that both the oxygen requirements of the microorganisms are covered and permanent sludge deposits are prevented. In contrast to biofilm systems, the multiplication rate of the microorganisms is much higher, so that the biomass can adapt more quickly to different levels of pollution. #Activated sludge treatment plants are not so sensitive in terms of sludge removal from the primary treatment. For this reason, more sludge can be stored in the primary treatment than in biofilm systems. Compared to activated sludge treatment plants, which work in a continuous flow process, the SBR process is less sensitive to larger amounts of water flowing in for a short time, as there is a buffer volume. An additional secondary clarifier is also not required, since there is no aeration during a limited phase and the activated sludge can settle undisturbed. After this phase, part of the cleaned water is pumped from the upper part of the system into the drain. After the pumping process has been completed, the space in the bioreactor that has become free is charged with the pre-cleaned wastewater from the #settlement pit. These plant systems can usually be easily retrofitted in existing #septic tanks.

What should be given special attention?
If the discharge point is higher than the sewage treatment plant outflow, there is no need for an additional pump shaft in systems with electrically operated pumps. Function, cleaning performance and energy saving are largely dependent on intelligent software. The function of the system is only guaranteed if all aggregates are functional. The number of built-in pumps, compressors and solenoid valves depends on the manufacturer. The number of electrical units is between two and five.

What are the controls like?
If all malfunctions are recognizable via alarms, only the legally prescribed operator checks need to be carried out.

CBR plants

#CBR plants are operated with a free-floating biomass. The microorganisms combine to form sludge flakes, which can have different sizes depending on the aeration method. In contrast to biofilms, the specialist can assess the condition of the system with the naked eye based on the color, shape, smell, quantity and settling behavior of the sludge flakes. The necessary aeration is generated with an air compressor and fed into the bioreactor via perforated membrane aerators. In wastewater treatment, effluent classes C (carbon degradation), N (nitrification) and D (denitrification) can be achieved.

Special system features
Due to their biomass structure, #CBR sewage treatment plants have a very good utilization of the reactor volume compared to biofilm plants. Due to the prescribed dimensioning of the bioreactor in small sewage treatment plants, these plants can process at least twice the dirt load for which they were designed. The ventilation and the possible additional hydraulic load must be adapted to these conditions. The strength of the aeration must be designed in such a way that both the oxygen requirements of the microorganisms are covered and permanent sludge deposits are prevented. In contrast to biofilm systems, the multiplication rate of the microorganisms is much higher, so that the biomass can adapt more quickly to different levels of pollution. Compared to activated sludge treatment plants that work in a continuous flow process, the CBR process is less sensitive to larger amounts of water flowing in for a short time, as there is a buffer volume. The built-in throttle orifice in the drain separator prevents the treated waste water from draining off prematurely. The usual sludge retention via a secondary clarifier is no longer necessary. This task is performed by the much smaller outflow separator, which is housed in the bioreactor. The total volume required is slightly less than that of an SBR plant. The excess sludge that occurs is drawn off by an automatic siphon before the start of aeration. The built-in #float switch determines the point in time for economy operation and regulates the #nitrogendegradation. In the event of disproportionately large amounts of water accumulating, an alarm condition is also triggered. The technical equipment is very small due to the system, so that the system control can be easily built. The only electrical unit is an air compressor, which ensures that the microorganisms are supplied with oxygen via a hose aerator in the reactor. These plant systems can usually be easily retrofitted in existing septic tanks.

What should be given special attention?
If the discharge point is higher than the sewage treatment plant outlet, the difference in height must be bridged with an additional pump. Simple system operation only requires the maintenance company to readjust the ventilation times (in the event of a change in the number of residents who are permanently present).

What are the controls like?
Only the statutory operational controls have to be carried out.

Systems with additional UV sterilization

The #UV disinfection is usually preceded by an #SBR treatment plant. With this plant constellation, the effluent classes C (carbon degradation), N (nitrification), D (denitrification) and H (hygienization) can be achieved for the first time. The systems are approved wherever sterilization of the waste water is required. Discharge in bathing lakes or in water protection areas are some applications.

Special system features
During the pumping process from the SBR plant, the water is guided through a system of pipes past a UV lamp that emits a wavelength of 254 nm. The water throughput is set in such a way that harmful bacteria are killed by the UV effect. Since the full power of the UV lamp is only reached after a burning time of 5 minutes, it is ignited in good time before the pumping process and then switches off again. Compared to the MBR system, the power consumption here is relatively low. The UV device can be housed in an outdoor cabinet or in the shaft of the sewage treatment plant. The service life of the lamp is - depending on the lamp type - several years. The tube of the UV lamp must be cleaned at every maintenance. Since no filter is used in this system, there is no risk of backwater when the cleaned waste water is discharged.

What should be given special attention?
The UV lamp should be attached in such a way that it can be easily removed for cleaning. A warning device for recognizing the timely replacement of lamps is mandatory. For optimal disinfection performance, the water flow rate must be throttled using a slide according to the manufacturer's specifications.

What are the controls like?
No.

That was the fifth and last part of our series "Everything about wastewater treatment and small wastewater treatment plants" - from the basics to individual cleaning processes and types of plants, from basic knowledge to processes and systems that are advisable in times of climate change.