FIELD OF THE INVENTION

The invention is in the field of wastewater treatment, in particular to a method of operating a wastewater treatment system for achieving rapid granulation in the start-up of a full-scale aerobic granular sludge system. The invention fur ther makes use of redox and phosphate measurements for con trolling the aeration rate of an aerobic phase during the treatment of wastewater. The invention can also be used to op timally control an aerobic granular sludge system for carbon and enhanced biological phosphorus removal whilst preventing nitrification and hence nitrogen removal.

BACKGROUND OF THE INVENTION

The invention is in the field of wastewater treatment.

Wastewater treatment typically involves several stages. During primary treatment, heavy solids may settle on the bottom of a basin and light oily materials may accumulate on the surface of the water. Heavy solids and light oils may be removed and the primary-treated wastewater may be subjected to a secondary treatment, wherein dissolved and suspended biological material may be removed. Secondary treatment may typically be performed using microorganisms that convert biological material being present into sludge. Typically, secondary treatment may in clude also the removal of pollutants such as phosphates and nitrates and may be combined with a tertiary treatment to fur ther polish the obtained treated wastewater quality. Secondary treatment of wastewater typically involves an anaerobic zone, an anoxic zone, and an aerobic zone, wherein wastewater is contacted with active sludge. The micro-organisms present in the sludge effectuate sludge growth, wherein organic matter is converted into sludge. Surplus sludge may be separated from the treated water by settlement and subsequently discharged from the wastewater treatment plant as waste. Recent improvements however, have managed to use the sludge as a valuable resource.

Prior to start-up of a biological wastewater treatment system, seed sludge is typically added to the reactors. The start-up period can be defined as the period from initial seeding and beginning to process wastewater, through to stable treatment capacity whereby all intended wastewater can be treated through the system. Such a start-up period typically involves acclimatization of the seed sludge and an increase in reactor sludge concentration up to a level that can treat all intended wastewater through the system. Such a start-up period is often referred to as process commissioning of a biological treatment system.

For aerobic granular sludge systems, granular seed sludge can be used for start-up, however sufficient granular seed sludge is often unavailable and expensive. An alternative for aerobic granular sludge systems is to start-up with flocculent seed sludge (activated sludge) , which is more readily availa ble, and to subsequently apply conditions that lead to granu lation - formation of granular sludge - as the start-up of the system progresses. This granulation process can take a signif icant period of time - circa 3 to 12 months - to go from a concentration of approximately 1-2 gMLSS/1 of flocculent seed sludge in the reactor to circa 6-12 gMLSS/1 of granular sludge (full design capacity of the system) .

Measuring or monitoring concentrations and other parameters during wastewater treatment is well established in the art. These parameters are measured via widely available instrumen tation in wastewater treatment processes. Using such instru ments to control biological treatment systems, and in particu lar to control the aeration input to a treatment system is well established.

Processes for treatment of wastewater are for example known from WO 2008/141413, wherein a mixture of wastewater and sludge, downstream of the anaerobic and aerobic zones, is led to a fast settler which operates discontinuously. Settled sludge is transferred to the anaerobic zone and the remainder of the mixture is led to a second settler. An alternative wastewater treatment system is described in US 2014/0144836, wherein part of the sludge formed in the process is subjected to a gravimetric selector. The heavy sludge is fed back to the process while the light sludge is subjected to post-treatment. The remaining part of the mixture of sludge and water is led to a settler and the sludge settled therein is recycled to the start of the treatment process.

AU 2011223510 describes a method for rapid start-up of an aerobic granular reactor, involving using fragments of granules extracted from another operational reactor. This method requires a source of granular seed sludge and is not practical for large full-scale reactors.

Yuan et al . in J. Hazardous Materials, Elsevier, Vol . 178, No. 1-3, p. 1041-1045 recites the effect of dissolved oxygen on nitrogen removal and process control in an aerobic granular sludge reactor, wherein aeration is stopped at the end of the process. ORP is measured to limit a duration of the process.

US 5,288,406 A recites a continuous process by which waste wa ter containing low to high concentrations of organic constituents and total nitrogen is biologically purified to such an extent that the organic constituents are minimized, the con tent of total nitrogen is completely nitrified, if it exceeds the assimilation demand, and the nitrate formed is reduced by aerating the waste water or exposing it to oxygen while not exceeding a preselected redox potential. In the subject pro cess, the expenditure on the oxygen introduction and the ex penditure on the denitrification are minimized. Further, the amount of excess sludge produced is extremely small. WO

00/37369 Al recites a single tank submerged membrane bioreac tor for treating feed water having unacceptable levels of am monia or total nitrogen has both a membrane scouring bubble supply and an oxygenating bubble supply.

A major drawback of these prior art processes is that none of them describe achieving rapid granulation in a full scale aerobic granular treatment plant. Furthermore, these systems do not achieve rapid granulation at the expense of limiting nitrification, which nitrification is a problem in view of un wanted chemical species being formed.

The present invention therefore relates to an improved method of operating a wastewater treatment system, which over come one or more of the above disadvantages, without jeopard izing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a method of operating a wastewater treatment system. The present method provides a speedy start-up of a biological treatment system, which is important (e.g. economically) to enable a new treatment system to provide full capacity within as short a time-frame as possible. For amongst others rapid start-up the present invention makes use a control system that favours rapid granulation in a full-scale aerobic granular sludge sys tem. The invention therefore provides a control system that can substantially shorten the start-up period of a full-scale aerobic granular sludge system.

It has been found that aerobic granular sludge may be formed and maintained in the present wastewater treatment sys tem by favouring slow growing bacteria, such as PAOs (polyphosphate accumulating organisms) , and GAOs (glycogen accumu lating organisms) , at the expense of OHOs (ordinary hetero topic organisms) which normally tend to grow more rapidly. For wastewater containing phosphorus (which is the case for a majority of domestic or municipal wastewaters, as well as for some industrial wastewaters) it has been found that PAOs are typically prevalent in aerobic granular sludge. The present invention provides a control system that optimally favours PAOs as a key means to achieving rapid granulation. It is found that PAOs may require a switch between aerobic and an aerobic conditions, preferably in a cyclic mode. Under anaerobic conditions PAOs take up volatile fatty acids (VFAs) and store the readily biodegradable Chemical Oxygen Demand (rbCOD) as storage polymers, whilst phosphate is released to gain energy to undertake the aforementioned VFA uptake. Conversely under aerobic conditions the PAOs use the storage polymers to gain energy and grow, whilst phosphate is taken up for the next anaerobic phase.

The invention focusses on aerobic granular sludge systems, which may apply a sequential batch process in a reactor, such as without a separate device for secondary clarification or settling. It is noted that fundamental elements of this con trol system may also be applied to continuous aerobic granular sludge systems. Furthermore, this invention is focused on start-up and subsequent control of full-scale reactors for the treatment of wastewaters. It is noted that the present control system could also be applied to laboratory scale or pilot scale reactors.

The invention may also be used as an optimal control system for a reactor used to generate seed sludge for other treatment plants and for a reactor where the primary goal is harvesting of excess or waste granular sludge as an input to a sludge beneficiation process, such as the extraction of biopolymers. The invention may be used on aerobic granular sludge systems and may also be applied for optimized control of any biologi cal phosphorus removal system, such as activated sludge systems based on Sequencing Batch Reactor or continuous flow technologies. The invention is therefore versatile.

The method comprises contacting the biomass in the system under anaerobic conditions with a biodegradable organic carbon load forming organically loaded biomass.

A core step of the present invention is that after loading the biomass in the system an aerobic phase (or aerobic step) of a wastewater treatment process is introduced, i.e. a mixture of wastewater and sludge is aerated, hence aerobic. Aera tion per se is known in the art and involves the introduction of an oxygen-containing gas in the system, typically in a re action zone. Any oxygen-containing gas can be used in this respect, but typical examples include oxygen, air, and mixtures thereof. Under such aerobic conditions, degradation of organic material occurs, and typically also ammonium, that may be pre sent in the wastewater, is converted into nitrogen oxides, such as nitrate and nitrite (nitrification) . Prior art

wastewater treatment processes are typically operated by con trolling the amount of oxygen that is introduced during the aerobic phase. Thus, the amount of oxygen being introduced is controlled by the oxygen concentration in the wastewater. Typ ically, a target value (or set point) of about 1-4 mg oxygen per L wastewater may be used. In case the oxygen concentration drops below this target value the aeration rate, i.e. the amount of oxygen that is introduced, may be increased; like wise this rate is decreased when the oxygen concentration rises above the target value. As such, sufficient oxygen is present during the entire aerobic phase, such that the desired biological processes occur. It has been found that high organic loading (such as in a low sludge age situation) of a biological treatment system, such as a wastewater treatment system, can lead to a loss of nitrification due to washout of nitrifiers. Furthermore, it is found that lower oxygen concen trations limit nitrification.

The present aeration is controlled and maintained by an inventive control system. The control system can also be used beyond start-up of an aerobic granular sludge reactor, to con trol said reactor to achieve optimal and stable biological phosphorus and carbon removal, whilst limiting nitrification. Such a control system could be applied for aerobic granular sludge systems where there are no nitrogen removal require ments, with the main advantage being optimized (reduced) aera tion use through the limiting of nitrification. In the control system oxidation reduction potential (ORP) and optionally phosphate content of the loaded wastewater are monitored. The ORP is maintained in a range of -50mV<ORP<50 mV by increasing or decreasing aeration, based on the monitored ORP and phosphate content, if relevant. It is noted that when loading the system the ORP may be lower than -50mV, but only for a short period of time, as a consequence of loading.

The present method comprises a control that leads to rapid granulation. One benefit of rapid granulation is the ability to complete the start-up period sooner and have the necessary capacity to treat all the intended wastewater. The present in vention of optimized control for PAOs and granulation has some key benefits in terms of aeration requirements. Nitrification is suppressed by high loading and redox based aeration control which limits oxygen input to levels lower than those required for extensive nitrification. Nitrification uses oxygen and hence by limiting nitrification less aeration is required, which is a further advantage. Furthermore, for stopping aera tion an optimal control is achieved and thereby preventing over-aeration, with associated high aeration energy require ments. The completion of P uptake may be an ideal stopping point for aeration, as this provides favourable condition for PAOs, and may be a point in the cycle when degradation of car bon is sufficiently complete to enable effluent discharge fol lowing a separation step. Furthermore, the present control method is found to prevent secondary P release, by stopping aeration as soon as P uptake is complete. Also by limiting the above nitrification the present control also limits NOx for mation. As such a limited competition for VFAs by denitrifiers during the feeding phase is achieved, i.e. PAOs can utilize nearly all VFAs in the feed wastewater.

Once a rapid start-up has been achieved and granulation is completed in addition N removal processes may be introduced i.e. a different control system may be applied subsequently, such as in a case when the system has N removal requirements. Alternatively, if the system is intended to only remove carbon (COD) , then the method may be continued to provide optimal control of the system, with e.g. optimal aeration energy usage as well as optimal biological phosphorus removal.

The present invention may make use of a number of control elements, such as applying a high organic loading, during an anaerobic feed phase, during a subsequent aeration phase control aeration using redox (ORP) , wherein aeration is adjusted accordingly. The ORP set-point is selected to supply enough air {oxygen} for PAOs to undertake their aerobic processes (use of storage polymers for energy generation and growth, as well as phosphate uptake) , and preferably to limit nitrification. The aeration phase may be stopped as soon as phosphate uptake is completed. Control of this stopping point may be achieved via a phosphate content measurement instrument, with an associated set-point. Also the stopping point can be based on the redox measurement. And stopping aeration can be based on when aeration reaches a predetermined set-point. It is noted that typically the ORP goes from low to high, a phos phate content goes from high to low, and aeration generally starts high and then goes lower and lower.

In summary inventors optimized process control for phosphorus accumulating organisms (PAO) . Inventors operate a system in an ORP windows, which is optimal for PAO' s and where these outcompete other organisms by a higher affinity for air; espe cially nitrifiers are found to need a higher ORP to be active. This range is typically in between -50 mV and +25 mV, prefera bly closer to +25 mV. It is noted that due to circumstance an ORP can be somewhat higher than +25 mV, but preferably only for a short period (<+ 30 minutes) . The present information provides information on growing/taking up phosphate of the PAO' s and if these are "fin ished" doing so. Inventors take into account activities and (average) influent composition, with as a result that the sys tem can run an aeration phase with a fixed duration. By meas uring oxygen consumption rate in the reactor it is determined that the process is finished. This could in principle also be done by another method for measuring activity of the sludge.

By measuring the phosphate concentration it can also be deter mined that the process is finished, such as if a phosphate concentration becomes below a set-point. Phosphate can be measured using a PHOSPHAX of Hach. By measuring K ⁺ concentra tion, which is typically a counter ion of phosphate in poly phosphate, a similar result is obtained, and likewise for Mg ²⁺ which is also a counter ion.

Thereby the present invention provides a solution to one or more of the above-mentioned problems.

Advantages of the present invention are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a method according to claim 1.

In an exemplary embodiment of the present method during aeration microbial sludge may be granulated, may be flocculate, may form a biofilm, or a combination thereof. In view of operating a granulated sludge is preferred, but other forms of sludge could be used as well.

In an exemplary embodiment of the present method steps (a), (b) , (c) and (d) are repeated, such as 2-10 times re peated, and (e) wherein sludge is separated, and/or removed, and/or settled, preferably after step (d) and before step (a). In these cycles aeration typically takes place during a period of 30 minutes to 12 hours, such as 60 minutes to 6 hours. By repeating a further optimization, e.g. in terms of startup, is achieved.

In an exemplary embodiment of the present method growth of poly-phosphate accumulating organisms (PAOs) during aera tion may be favoured. Thereby the built up of sludge is fa voured significantly. In an exemplary embodiment of the present method available nitrogen may be up taken by microorganisms during aera tion, such as nitrogen from ammonia, an organic nitrogen source, and urea. Therewith the amount of nitrogen in the wastewater is reduced and nitrification is prevented.

In an exemplary embodiment of the present method (e) sludge may be separated from the wastewater such as identi fied above. This might be advantageous, for instance if the sludge finds a further application, or if surplus sludge is removed for proper functioning of the reactor.

In an exemplary embodiment of the present method { f ) the aerated wastewater may be treated further. The present method may be considered to provide initial steps in

wastewater treatment; the as such pretreated wastewater may be treated further, for instance if required, such as by an anaerobic process generating methane.

In an exemplary embodiment of the present method the ORP may be maintained in a range of -25 V<ORP<30 mV, preferably 0mV<ORP<25 mV, even more preferably lmV<ORP<24 mV. It has been found that these narrow ranges provide the best re sults. It is noted that when the aeration starts the ORP may be relatively low and it will generally take several (30-60) minutes for the ORP to increase to levels above -50 V and thus for the PAO' s to become active. The ORP in the process control may become higher than +25 mV, but duration thereof is preferably limited {e.g. to less than 1-30 minutes) to ensure nitrifiers don't grow into the sludge. Sometimes ni trification is only limited to a certain extent, and not completely prevented. In this case the ORP range may be stretched somewhat, such as up to +50 mV.

In an exemplary embodiment of the present method aeration may be provided in at least one of a zone of the sys tem, in a phase of operation of the system, and in a reactor of the system. There is some degree of operating freedom in this respect; the goal is to aerate the system appropriately, whereas the precise location is found to somewhat less relevant.

In an exemplary embodiment of the present method the system may be loaded with > 0.5 kg COD, preferably > 1, more preferably >2, per m ³ system volume/day.

In an exemplary embodiment of the present method phos phate content may be monitored by at least one of a PO4-P content, a Mg content, and a K content. It has been found that also Mg- and K-content, in addition to PO4-P content, are good indicators of the amount of available phosphate being present. It is considered that as phosphate salts may be formed, or could be formed, the relevant cations, in this case Mg and K, provide indirect information to the phosphate content .

In an exemplary embodiment of the present method the set-point of phosphate content may be 20% of an initial value, preferably 10% of the initial value. It is noted that initial values may vary widely, depending on a source of wastewater. For instance for industrial wastewater the phos phate content may be 100 mg/1, whereas for municipal

wastewater it may be 10 mg/1.

In an exemplary embodiment of the present method the set-point of phosphate content may be 1 mg PO4-P/I, preferably 0.5 mg PO4-P/I, such as 0.05-0.4 mg PO4-P/I.

In an exemplary embodiment of the present method the set-point of stopping aeration may be 25%, preferably 20%, such as 10%, relative to a maximum aeration. By closely mon itoring the aeration required, relative to for instance a maximum amount, information on the progress of starting up may be obtained. It is found when the aeration has dropped to below 25% it may be considered that the start-up is com pleted. To be on the safe side, one may wait somewhat longer, until the aeration has dropped to 20%, or even 10%.

In an exemplary embodiment of the present method the aeration rate may be varied in the range 0.05-1 kg 0 ₂ /m ³ /h, preferably 0.1-0.5 kg 0 ₂ /m ³ /h. This range is comparable to a range of 0-1 m ³ O2 per m ³ reactor volume per hour (at 25 °C and 100 kPa) , preferably 0.05-0.5 m ³ 0 ₂ , more preferably 0.1-0.4 m ³ O2, even more preferably 0.2-0.3 m ³ O2. So rela tively small amounts of oxygen are found to be sufficient already .

In an exemplary embodiment of the present method aera tion may be performed by introducing an oxygen-containing gas, preferably air, in a reaction zone comprising the mix ture. Instead of pure oxygen also oxygen containing fluids may be used, such as air, which is typically easier.

In an exemplary embodiment of the present method

the aeration rate may be adjusted by a conventional or advanced controller type, such as a PI, a PID, a genetic algorithm, a neural network, and a fuzzy logic controller. Fuzzy logic may refer to logic that can handle concepts as "true", "false", and in addition "partially true". Genetic algo rithms and neural networks can perform just as well as fuzzy logic in many cases, though fuzzy logic may have as ad vantage that user experience can be used in the design of the controller. This can make it easier to mechanize tasks that are already successfully performed by humans.

In an exemplary embodiment of the present method the system may be operated batch-wise, continuous, semi-continu ous, cyclic, plug-flow, or a combination thereof.

In an exemplary embodiment of the present method the wastewater may be pre-treated prior to step (a) , preferably by at least one of clarification, grit removal, fat removal, grease removal, pH-adj ustment , and pre-sedimentation.

In an exemplary embodiment of the present method the organic loading rate may be determined without considering the anaerobic cycle time in the case of a batch wise system.

In an exemplary embodiment of the present method the method is incidentally used in the operation of the

wastewater treatment to accelerate start-up and/or to improve granular characteristics of the biomass, by applying and maintaining the method during a period of time of less than 12 weeks, preferably less than 6 weeks, such as less than 2 weeks.

In an exemplary embodiment of the present method >3 kg sludge/m ³ wastewater may be present, preferably >5 kg sludge/m ³ , such as >8 kg sludge/m ³ .

In an exemplary embodiment of the present method the system may be a full-scale system, a laboratory scale reac tor, or a pilot scale reactor, and may be selected from at least one of an aerobic granular sludge system, such as a Nereda system. In a second aspect the present invention relates to a use of the present method for achieving rapid granulation in the start-up of a full-scale aerobic granular sludge system, or for control of an aerobic granular sludge system for car bon and enhanced biological phosphorus removal, or for lim iting nitrification and hence nitrogen removal, or a combi nation thereof. It is noted that nitrification may be partly prevented, such as for 50-90%, depending on further boundary conditions, mode of operation, etc.

The invention is further detailed by the accompanying figure and example, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many var iants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims .

SUMMARAY OF THE FIGURES

Figure 1-4. Comparative Examples.

DETAILED DESCRIPTION OF THE FIGURES

Figure 1 shows a plot of granulation (vertical axis, rela tive to a final stage) versus time for a system according to the invention (A) and systems according to the prior art (B) . The present system reaches a level of 40% within 25 days, a level of 60% in about 50 days, and a level of 70% in about 70 days, whereas the prior art systems reach a level of about 20% after 100 days, a level of 40% within 250 days, a level of 60% sometimes in about 300 days, and typically do not reach a level of 70% within the time frame of the measurements. Hence the present system reaches high levels much quicker and also reaches higher levels within the time frame of about a year. Such is considered a significant economical and production benefit .

The figures are further detailed in the description and ex amples below.

EXAMPLES/EXPERIMENTS

The below relates to an example of the present invention.

The startup of a pilot-scale Nereda with a working volume of 1000 m ³ was studied. This Nereda reactor is located in

Utrecht, the Netherlands. The Prototype Nereda Utrecht (PNU) is run with real wastewater, and its effluent is recirculated to the conventional wastewater treatment plant so there is no formal minimal effluent quality to be met.

The influent and effluent streams of the PNU were sampled with an autosampler and multiple probes were installed in the Nereda: an AN-ISE probe for ammonium and nitrate measurement, a PHOSPHAX for P0 ₄ -P measurement, an AMTAX for ammonium meas urement, a SONATAX for sludge blanket level measurement, and a SOLITAX for dry solids measurement. These probes collected online data. During normal operation, influent and effluent were also sampled via an autosampler and the 24-hour

samples were analyzed.

For startup, the reactor was seeded with one truckload of thickened activated bio-phosphate-removing sludge from de

Bilt, the Netherlands. The thickened sludge had been dewatered through treatment with polyelectrolyte solution. After seeding, the reactor was aerated at maximum capacity for 24 hours to break the polyelectrolyte bonds. The following recipe was then applied:

1: Feeding phase

- Influent feeding at 225 m ³ /h until a batch size of 200 m ³ was reached. The feeding time was 53 minutes.

2: Aeration phase

- The water level was lowered from 7.1 to 6.9 meters.

- Aeration took place for at least 30 minutes. Stop condition: P0 ₄ -P 0.5 mg/L or lower OR until 700 minutes elapsed OR a redox value 70 mV.

3: Settling phase

- Aerate for 5 minutes to mix the reactor

- Settling: 22 minutes

- Sludge discharge: discharging until the level is 6.45 m.

PNU sludge was sampled daily during the first aeration phase of the working day. This was done after minimally 15 minutes of aeration to ensure that a well-mixed sample would be obtained. Sludge settling was monitored over sixty minutes by sludge volume (SV) tests. 1 liter of sludge was added to a graded 1 liter measuring cylinder. Size distribution of solids in sludge was determined through sieving of 1L of sample over four sieves with sieve diameters of 200 micrometer, 400 micrometer, 630 micrometer and 1 millimeter. The sieves were stacked in order of smallest to largest, with the smallest di ameter sieve at the bottom of the stack. 1L of sludge was then poured over the stack and each sieve was washed with tap wa ter. The fractions caught in each sieve were transferred to weighed aluminum drying dishes and dried overnight at 105 degrees Celsius.

In general, over time the dry solid contents in the PNU reactor accumulated and the phosphate removal capability of the biomass increased. Ammonium concentrations were high, this was expected as the nitrification and denitrification processes were limited to achieve a focus on phosphate removal.

In the first week of operation, the length of the cycle was short, approximately 2.5 hours, due to a short aeration time.

Each day of the first week, therefore, nine to ten cycles were completed. This was caused by a low PO4-P concentration, which caused the aeration stop condition of 0.5 mg/L PCh-P to be reached quickly. As the PAO presence and the phosphate accumu lating capacity of the PNU sludge increased, the length of the cycles also became greater. The cycle length increased to ap proximately four hours in the second month of startup. In gen eral, during the start-up approximately 8 cycles were com pleted per day. The average sludge loading rate for the days when COD measurements were taken was calculated to be 0.27 (±0.12) kg COD/ (kg TSS day). The sludge volume index is determined by dividing the sludge volume (SV) after 5 and 30 minutes by the dry solids concentration.

The accumulation of large particles in the PNU showed a linear increase for the first two months. The sludge changed from flocculent in the first weeks to granular after 5 weeks, with a subsequent increase in size.

So during the present startup of the PNU, a different strategy than conventionally applied for startup was used. As described this strategy entailed a focus on bio-P removal and limiting of nitrification and denitrification processes. In contrast, prior art startup strategies focus on achieving max imization of both P and N removal. Startups are usually seeded either with discharge sludge from fully started-up Nereda re actors or with conventional activated sludge. The present startup was seeded specifically with bio-P activated sludge, increasing the likelihood that PAOs would be active from the start .

From Figure 1 it can be seen that by far the highest granule content in the first three months was achieved for the present startup of the PNU . Figure 1 compares the granule content for four startups in three locations over the first year of startup. The granule content data points for all four startups can be fit with exponential curves. The result again shows that the present PNU startup achieved high granule con tents in a much shorter time span. The three other prior art startups show similar exponential trends, with slow growth of granules until day 100-200 and faster increase after this point. The present PNU trend shows such accelerated granule growth from the start. So the strategy with focus on bio-P re moval and limitation of nitrification and denitrification pro cesses achieved a higher granule content in a shorter time than startups applying the conventional startup strategy of both N and P removal.

In summary, within one month of startup, granules with a diameter of >630 pm were observed in the reactor and an almost linear increase in granule-sized solids was recorded over the first two months of startup time. This result was compared to that of previous startups of the PNU. An earlier start up that did not apply the bio-P removal focused strategy, but a con ventional strategy with a focus on both phosphate and nitrogen removal, achieved a much lower rate of granulation in the first months. A further comparison was made to other startups that had applied the conventional startup strategy. In all cases, the current PNU startup achieved the highest granule percentages in the shortest time.