Performance of nitrogen removal in attached growth reactors with different carriers

Two waste materials, concrete and sponge, were used as biomass carriers in the attached growth reactor in a nitrogen wastewater treatment system. The nitrogen removal performance was compared to a control reactor using commercial carrier material. The highest nitrogen removal efficiency, 87%, was found in the sponge reactor, with the concrete reactor showing 82% efficiency ahead of the commercial reactor of 76%. A thick biofilm developed on the fiber of the sponge carrier, with the biomass increasing from 270 g-VSS/m 3 -carrier to 1,000 g-VSS/m 3 -carrier. For the concrete carrier, biomass was observed on the concrete cracks and also as a biofilm on the surface. The maximal biomass was 630 g-VSS/m 3 -carrier. The content of the biomass agglomerated in the commercial carrier was 310 g-VSS/m 3 -carrier. Nitrification and denitrification simultaneously occurred to remove nitrogen in the sponge and the commercial carrier reactor. However, in the concrete reactor, nitrification mainly occurred during the aeration phase and denitrification occurred in the non-aeration phase. These results demonstrate that the sponge was the best carrier, with high nitrogen removal efficiency, dense biomass and tolerance to shock loading. The simplicity inherent in the system design together with good performance make it suitable for use in wastewater treatment systems.


INTRODUCTION
The removal of nitrogen, including ammonium-nitrogen (NH 4 -N), nitrite-nitrogen (NO 2 -N) and nitrate-nitrogen (NO 3 -N), from domestic wastewater is an important issue for water resource management, especially for controlling polluting water runoff with a high nitrogen content. High nitrogen levels in wastewater discharged into local water sources, such as ponds and small catchments, common in rural areas, can often result in toxic blooms of algae and 'scum' on water surfaces, rendering it unfit for fish breeding, crop watering and animal consumption, important uses in rural areas. Domestic wastewater usually has high nitrogen content and is an easily identifiable source as well as being easily treated at the local level.
Several physical and biological treatment methods have been developed to remove the nitrogen from domestic wastewater. These treatment methods include granular nitritation and anammox in membrane-aerated biofilm reactors (Li et al. ), hybrids of activated carbon and sequencing batch reactors (Sirianuntapipoon & Chairattanawan ), and combined subsurface and surface flow constructed wetlands (Sartori et al. ). Current treatment systems are mainly focused on the modification of exiting traditional systems to enhance their treatment ability with co-removal mechanisms. However, more importantly in remote areas, a reliable supply of clean water is an imperative, and therefore reliable domestic wastewater treatment plants of simple design, using uncomplicated materials available locally, with low operating costs, to remove the nitrogen from the wastewater, are essential. The attached growth reactor is a type of biological treatment plant, in which the active sludge is attached to a carrier as a biofilm, is such a reliable technology appropriate to remote areas. can provide the soluble organic carbon for denitrification in wastewater containing a low C/N ratio (Chu & Wang ).
Although there has previously been extensive research on nitrogen removal performance, and microbial communities, in attached growth reactors, with a variety of carriers, the use of waste material as the biomass carrier for biomass attachment to enhance the nitrification and denitrification has not been comprehensively explored. Concrete is a construction waste product which is mostly disposed by open dumping in landfills or otherwise illegally dumped. The surface roughness and high voids of concrete make it potentially useful as a biomass carrier. Another waste product, sponge, probably used in all households in the nation, is a porous material appropriate for biomass attachment, inexpensive and readily available, even in remote areas.
Prior to this, carbon in various forms has been used for nitrogen removal from domestic wastewater. This is because the wastewater has a low carbon content which is insufficient for complete denitrification. In previous studies, the carbon concentration was maintained in the range of 500-600 mg/L, and the C/N ratio was around 25 (Feng et al. ; Liu et al. ). Although excellent performance of nitrogen removal has been attained in previous studies (Walters et al. ), the residual carbon can impact the quality of the treated water. Further, when the biodegradable material acted as carrier and carbon source, the amount of carbon release and stable release rate were matters of concern.
Therefore, the aim of this study was to investigate the possibility of using alternative materials for domestic wastewater treatment in the attached growth reactor. Concrete and sponge were considered, for the reasons previously stated. The nitrogen removal performance at low carbon content levels of an attached growth reactor, using variously the concrete and the sponge material as the carrier, was investigated. The performance of these materials was compared to that of a commercial carrier which is commonly used in attached growth reactor wastewater treatment plants. The removal mechanisms occurred in the different carriers was clarified, and the attached biomass was observed for volume and time of growth.

METHODOLOGY Carrier
Concrete and sponge are two waste materials which were used for biomass attachment in this research. The concrete which came from a construction site and sponges from a local market were cut into 2 cm cubic shape. The commercial carrier with 2 cm diameter was used as a control to compare the performance. The carriers were transferred to a suspended sludge which had been cultivated by NH 4 -N feeding for a month. The accumulated biomass was in the range of 270 g-VSS/m 3 -carrier for the concrete and sponge carriers and 180 g-VSS/m 3 -carrier for the commercial carrier.   The C/N ratio was kept at 2.5 in Phase 1 and increased to 3.5 in the sequencing phases (in Table 1).

Biomass measurement
The carriers were taken from the attached growth reactors at the initial stage, day 0, and day 45 and 70 (the steady state day). The biomass was removed from the carriers by rinsing and shaking for 15 minutes, and the liquid was then centrifuged and filtered. The mixed liquor suspended solids and mixed liquor volatile suspended solids concentrations in the biomass samples were measured according to the standard method (APHA ). The biomass content was calculated as g-VSS/m 3 -carrier. In addition, the carrier and its biofilm were observed by using a scanning electron microscope (Leo 1400 Series).

Analytical method
The performance of the attached growth reactors was NH 4 -N removal efficiency (%) Nitrogen removal efficiency (%) (2) Specific nitrogen removal(mg N=g VSS)

Performance of attached growth reactors
At the low NH 4 -N loading in Phase 1, the nitrogen removal efficiency for the three attached growth reactors increased continuously until it became stable at the steady state on day 45. In the steady state, the sponge reactor showed the highest nitrogen removal efficiency of 68%, followed by the concrete reactor of 61% and the commercial carrier reactor of 56% ( Figure 3). However, the NH 4 -N removal These results show that nitrification is more effective than denitrification for oxidizing NH 4 -N to NO 3 -N. The low concentration of organic carbon in the effluent indicated that the nitrification and denitrification was limited by the rate of denitrification due to the inadequate supply of carbon.
In Phase 2, the nitrogen removal efficiency continued to increase at the higher C/N ratio of 3.5. At the steady state, the nitrogen removal efficiency achieved 82% for the concrete reactor, 87% for the sponge reactor and 76% for the commercial carrier reactor. Similarly, the NH 4 -N removal efficiency also increased and reached 90-95% for all reactors. The dramatic increase in nitrogen removal efficiency was due to there being sufficient carbon available for microbial metabolism to remove the nitrogen, as indicated by there being residual carbon in the effluent. In addition, the growth of the biomass on the carriers was another significant reason for the dramatic increase in nitrogen removal efficiency.
The biomass content increased from 270 g-VSS/m 3 -carrier in the initial stage to 610 and 1,000 g-VSS/m 3 -carrier for the concrete and sponge at the steady state, and increased from 180 g-VSS/m 3 -carrier to 290 g-VSS/m 3 -carrier for the commercial carrier ( Figure 4). It can be seen that dense biomass was observed in the sponge and concrete carriers, which has a high porosity and particular surface characteristics which encouraged the faster attachment of biomass, whereas the commercial carrier has a smooth and rigid surface not as conducive to biomass attachment.
At the high NH 4 -N loading of 80 mg/L in Phase 3, the nitrogen removal efficiency suddenly dropped and then increased again to 80% for the concrete reactor, 88% for the sponge reactor and 70% for the commercial carrier reactor.
The nitrogen removal efficiency of each carrier at steady state showed that the nitrogen removal efficiency values at the high NH 4 -N loading were very similar to those at the low NH 4 -N loading. This means that all three reactors can effectively operate at either low or high NH 4 -N loadings. In addition, the biomass content slightly increased to 630, 1,030 The results clearly demonstrate that the waste materials of concrete and sponge can be used as effective and efficient biomass carriers in a biological nitrogen treatment system. The effectiveness of nitrogen removal was in the following order: sponge > concrete > commercial carrier. This variation in nitrogen removal which was related to the porosity and surface roughness. Moreover, the sponge carrier shows tolerance to high NH 4 -N loading and retains excellent performance.

Characteristics of biomass
During operation, the biomass developed as a biofilm covering the carrier. The biofilm on the sponge carrier grew rapidly from 270 g-VSS/m 3 -carrier at the initial state to 1,000 g-VSS/ m 3 -carrier in 45 days. The biofilm on the commercial carrier In contrast, the commercial carrier was made from plastic and contained large cavities which negatively impacted on the biomass growth and strength of attachment. The concrete carrier had a rough surface, which was appropriate for biomass attachment, but the rigid structure of the material prevented the growth of biomass inside the concrete. Similarly, simultaneous nitrification and denitrification was found in the commercial carrier, as indicated by slightly changing NH 4 -N and NO 3 -N concentrations. As discussed above, the high oxygen on the commercial carrier surface induced nitrification to occur in the outer microbial floc, and the low oxygen from diffusion resulted in denitrification in the inner floc. From all results, the concrete and sponge carriers can perform well in removing nitrogen from wastewater; however, the mechanisms of nitrogen removal were significantly different which affected the operating conditions required to achieve effective reactor performance.

CONCLUSIONS
Concrete and sponge waste materials can be used as the biomass carrier in nitrogen wastewater treatment. The performance of both in attached growth reactors was better than the use of a commercial carrier which is commonly found in wastewater treatment system. The highest nitrogen removal efficiency of 87% was found in the sponge reactor: this is due to the high biomass attachment of 1,030 g-VVS/ m 3 -carrier. The efficiencies of concrete and commercial carrier reactors were 82% and 76% respectively. The porosity and surface roughness of the carriers impacted on biomass attachment and mechanism of nitrogen removal. The biomass of 630 g-VSS/m 3 -carrier was mainly found on the surface of the concrete carrier, thus the concrete reactor requires intermittent aeration to obtain nitrification and denitrification.
On the other hand, the biomass attached in the inner cavities of the sponge and agglomerated as microbial floc in the commercial carrier, therefore simultaneous nitrification and denitrification can occur without a non-aeration period.