Appendix 20 Assessment of Beca s Report on Solid Settling Options

Transcription

1 Appendix 20 Assessment of Beca s Report on Solid Settling Options

2 Page 1 From: To: Selva Selvarajah, ENVIROKNOWLEDGE Ltd Wolfgang Kanz, Gisborne District Council Date: 30 June 2017 Topic: Assessment of CH2M Beca s report on solid settling options Summary Solid settling options in CH2M Beca s memo to GDC were assessed for their technical risks. Both circular and lamella clarifiers are considered as suitable to remove GDC BTF effluent solids. Since lamella clarifiers are capable of processing same BTF effluent loading within 85% less footprint than that is required by circular clarifiers, lamella clarifiers are considered superior. Information assessed: Gisborne WWTP Stage 2 Upgrade Memo 5 by CH2M Beca 26 April BTF Solids Settling Options Discussion Assessment of sludge and effluent characteristics from the Gisborne Biological Trickling Filter (BTF) and recommendations for wetland sludge treatment By NIWA September 2015 NIWA Wetland trial spreadsheet SP1 which has BTF effluent data from 23 Feb 2016 to 2 August 2016 following solid removal by NIWA conical method GDC BTF wastewater quality data collected during NIWA wetland trials from 22 May 2013 to 10 August 2016 GDC BTF data spreadsheet for BTF before and after wastewater quality from 6 January 2011 to 27 December 2016

3 Page 2 BTF Solids: Much of the BTF solids is produced from the sloughing process of bio-growth (which could contain dead and live microbes, fungus and protozoans) within the BTF filter media hence can be relatively large and heavy. Consequently, the BTF solids tend to settle faster because particle settling is independent to other particles with discrete settling motion. In contrast, the solids from an activated sludge system flocculate by aggregation of particles hence the settling may not be as fast as that in the BTF treated effluent. BTF effluent solids can be removed by filtration, clarification and ponds. In this memo, only clarifiers and ponds are assessed as in CH2M Beca s memo. Conventionally, clarifiers are used to remove solids from water and wastewater. In wastewater treatment, clarifiers are used as primary or secondary clarifiers. In continuous flow clarifiers, surface overflow rate (SOR) (m 3 /m 2 /d or m/d) or settling velocity (v c) (m/s) is one of the critical factors. NIWA s trials at GDC indicated that settling velocity of the BTF solids was m/s which was considered as ~15-50 times faster than that typical of activated sludge from Hamilton and Danish WWTPs (~10-5 m/s). However, NIWA found the specific resistance to drainage (SRD) of BTF sludge (~ m/kg) was about 2-15 times higher than that of activated sludge ( m/kg). The above observation suggests that while BTF solids can be removed easily, the dewatering of the BTF solids can be relatively slower. The mean total suspended solids (TSS) estimated in the BTF effluent assessed between 2011 and 2016 was 154 g/m 3 with BOD being 82 g/m 3 (Figure 1). The 95 th percentile value for TSS was 360 g/m 3 and BOD was 170 g/m 3. Based on the influent volume, the estimated mean TSS output from the BTF was 1.9 t/d (154 g/m 3 x m 3 ) and BOD was 1 t/d. NIWA s assessment of the TSS production was 2.2 t/d and BOD was 1.2 t/d. NIWA identified that the summer TSS was output was slightly greater (2.4 t/d). Based on CH2M Beca s data of average BTF effluent outflow of m 3 and average level of 190 TSS g/m 3 the TSS output was 2.7 t/d and BOD (with 100 g/m 3 ) was 1.4 t/d.

4 27/07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/2015 Page 3 Figure 1. BOD and TSS in BTF Effluent (g/m 3 ) BOD (blue) and TSS in BTF Effluent /01/2011 6/01/2012 6/01/2013 6/01/2014 6/01/2015 6/01/2016 Series1 Series2 Linear (Series1) Linear (Series2) Based on CH2M Beca data (Table 2 in CH2M Beca s solid removal options memo), the 95 th percentile TSS was 320 g/m 3 and daily BTF outlet flow was 39,900 m 3. The average BTF outlet flow was 14,400 m 3 /d and the average TSS level was 190 g/m 3. When estimating TSS mass output at 95 th percentile, it is not a matter of multiplying both 95 th percentile values of TSS and outlet flow because TSS levels are not dictated by the outlet volumes. Generally greater the outlet flow, lower the levels of TSS. Figure 2 shows the flow and TSS levels are not related. Figure 2. TSS level (g/m 3 ) and wastewater flow in m 3 /100/day Chart Title Series1 Series2

5 27/07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/ /11/ /01/ /03/ /05/ /07/ /09/2015 Page 4 It is noteworthy that given the minimum flow required to maintain BTF wetting was 32,400 m 3 /d (or 375 L/s), it is misleading to use raw influent volumes as BTF outlet flow volumes. This is because, a portion of the BTF effluent will be mixed with the raw wastewater influent (recirculated) to maintain the wetting volume. For the above reason, any such data analyses require access to actual BTF effluent output volumes. Using CH2M Beca s average flow of 14,400 m 3 /d and TSS level of 190 g/m 3, the average TSS output is estimated as 2.7 t/d. In comparison, NIWA s summer output estimate was 2.4 t/d. Using the inflow volumes provided in the GDC spreadsheet, Figure 3 shows the trend of TSS output. Figure 3. Daily TSS level output estimated using raw influent volume (kg/d) TSS daily output in BTF effluent (kg) Options assessed by CH2M Beca: Key points before assessing solid settling options Key Point 1 My assessment of CH2M Beca s memo is based on technical risks associated with the proposed options. My assessment does not include pricing/costs of installation and maintenance of the system and system specifications (e.g. clarifier surface area required, hydraulic loading rate etc.).

6 Page 5 Key Point 2 One of the key missing information in CH2M Beca s assessment is the solid removal efficiency of each option. Solid removal efficiency of a secondary clarifier depends primarily on the type and treatment of effluent, option used and design. Some options have inherent higher solid removal efficiencies. However, solid removal efficiency of a particular option can be improved by design and by optimising for key factors such as surface overflow rate, hydraulic retention time (HRT) etc. Whilst solid settling velocity can vary between effluents from different treatment systems (e.g. activated sludge treated effluent vs BTF effluent), it could also vary between two similar BTFs from different locations. For the above reasons, it is difficult to extrapolate solid removal efficiencies within a similar option (e.g. circular clarifier). Technically, similar solid removal efficiencies may be achieved within an option for different locations with slightly differing designs (e.g. surface overflow rate, HRT etc.). NIWA with its preliminary BTF effluent solid removal trials at the GDC site had shown that by using a simple solid removal method they could remove 68-95% of the solids after only one hour of settling, with majority of the settling occurring within 10 mins. NIWA trial also showed that solid removal resulted in a substantial reduction in BOD (72%), TN (48%) and TP (55%). One of the main reasons for NIWA s successful solid removal observation may be the use of short depth containers for solid settling trials (450 mm depth Imhoff cones with sampling at midpoint). Notwithstanding this, it is well known that BTF solids can be easily settled. On this basis and based on NIWA s trial, it can be expected that any good option should be able to remove 80-90% of the BTF effluent solids following clarification. I note in its solid removal options memo conclusion section CH2M Beca had a target TSS level of <30 g/m 3. On the other hand, NIWA s solid removal trial had shown that a mean TSS level of 6.4 g/m 3 can be achieved (Table 2 ENVIROKNOWLEDGE June 2017 Memo to GDC on Wastewater Quality of the Current BTF). Using CH2M Beca s average BTF effluent

7 Page 6 TSS level of 190 g/m 3 and target TSS level in the clarified BTF effluent of 30 g/m 3 I presume CH2M Beca would like to achieve at least 84% solid removal efficiency. Considering NIWA s solid removal trial outcomes and existing clarifier technology, CH2M Beca s target of <30 g/m 3 (or 84% solid removal efficiency) is technically achievable. Key Point 3 If GDC decides to use double BTF, the solid settling characteristics of the double BTF effluent and the total amount of solid treated may be different to that of the current single BTF effluent. Double BTF has the ability to reduce solid output by greater oxidation of COD/BOD thereby reducing the total solid outputs. For the above reasons, any designing or installation of clarifiers have to be performed well after the double BTF installation and operation to assess solid loading and characteristics. Key Point 4 I have not reviewed CH2M Beca s assessment of the filter option to remove solids from BTF effluent for the preparation of this memo. Filtration is a well-known technology which is known to achieve dual purposes of solid and microbial (mainly bacteria) removal, thus could eliminate the UV treatment step altogether. However, filtration footprint and running cost could be much greater than that of the clarifiers. If GDC considers filtration option and costing, the cost of UV system has to be subtracted. Key Point 5 As stated, TSS mass output estimates must be made using actual BTF outlet volumes, preferably discharge volumes. During the preparation of this memo, I did not have access to the above data. The BTF outlet volumes used by CH2M Beca appear to be similar to the influent volume data provided in the GDC spreadsheet. Given the BTF wetting requirements, the outlet volumes can be expected to be in excess of 30,000 m 3 /d. If CH2M Beca s outlet volume data were correct, I presume the BTF effluent recirculation is not performed as required by the BTF specifications.

8 Page 7 Key Point 6 The final key point is that solid removal does result in substantial consequential reduction in other contaminants such as BOD, TN and TP. The consideration of the above key contaminants removal must be one of the key purposes of using clarification as a treatment process, in addition to reducing solids to improve effluent clarity to increase UV treatment efficiency. Ponds Many TLA wastewater treatment sites still use facultative ponds to treat sewage wastewater. However, using ponds particularly facultative ponds for removing BTF solids is a backward step. As identified by CH2M Beca, ponds require large area (in hectares) and are technically risky. The only advantage of using ponds is solids can be held and treated in such ponds for many years without having to dewater/treat and dispose. However, the technical risks of using ponds to store and treat solids outweigh this single advantage. GDC BTF solids hold substantial amount of biologically degradable-c which is g/m 3. The daily output of BOD alone is t/day. Facultative ponds promote both aerobic and anaerobic digestion of the solids. Whilst aerobic digestion is favourable which does not result in odour, uncontrolled anaerobic digestion is temperature dependant, which could cause only partial anaerobic digestion under New Zealand conditions. This will result in considerable BOD accumulation in ponds. Under such conditions, prolonged warm conditions can trigger the release of methane and hydrogen sulphide. The only way to avoid such a situation is by extensive mechanical aeration. As identified by CH2M Beca, facultative ponds can also promote algal growth. Whilst the presence of algae can promote slightly better BOD breakdown by oxygenation during the daytime, management of algae can be problematic from the viewpoint of controlling TSS levels in discharge and unwanted high algal activity. The other factor which was also identified by CH2M Beca was the presence of large open water which would attract flying birds to the area, which could increase the risk of bird strike for the local airport operation.

9 Page 8 For the above reasons I will not recommend ponds as a BTF solid removal option. Circular clarifiers Circular clarifiers have been used widely in New Zealand to remove sewage effluent solids from secondary treated effluent. Circular clarifier installation and operation are well engineered and understood. The influent wastewater flow is radial from the centre to the wall edges. The bottom of the circular clarifier is slightly conical to enable sludge collection and removal. Circular clarifiers can be used to remove solids from both activated sludge and BTF effluents. In the case of activated sludge systems, filamentous bacteria interference with flocculation can be an issue. In the case of BTF effluent solids heavier solid particle size and discrete particle motion allow solid settling faster and less complex than that achieved from the activated sludge effluent solids. As identified by CH2M Beca, this option uses substantially lower footprint than that of ponds but greater than that of the lamella clarifiers. Circular clarifier option is substantially less risky compared to ponds. However, like any clarifiers, circular clarifiers designing is critical to the effective removal and management of the BTF effluent solids. Such solids require removal without causing anaerobic conditions. Longer HRTs could cause anaerobic conditions within the accumulated sludge which could affect the settling and dewatering characteristics of the solids. My recommendation is circular clarifiers can be considered as a safe and effective option to reduce BTF effluent solids, without giving regard to cost and footprint. Lamella clarifiers Lamella or inclined clarifiers have been used to treat water, wastewater and storm water to remove solids or sediments. There have been numerous commercial developments and promotion of such clarifiers in the past two decades. This technology reduces the travel distance of solid particles by housing several layers of inclined plates, thus reducing the HRT (hydraulic retention time) and required footprint substantially.

10 Page 9 Influent enters through an inlet pipe, and the plates through the side openings where the liquids and solids are then separated. The liquid flows upwards, whereas the solids settle on the plates and slide down into the hopper. One of the operational issues could be accumulation of solids in the hopper which could cause solids re-entering treated liquid. This can be avoided by automation which could trigger hopper emptying at the correct time. Lamella clarifiers are impressive in using less footprints. By CH2M Beca assessment, two lamella clarifiers require 85% less footprint than that required by two circular clarifiers. There are also tube settlers which use the same principle as lamella clarifiers. Overall, I concur with CH2M Beca recommendation of the use of lamella clarifier to remove BTF effluent solids. Ballasted sedimentation This is a hybrid system between flocculation and lamella plate settling process. Microsand and polymers are used to accelerate solid flocculation and solids are settled on the inclined lamella plates. Avoiding the use of chemicals including polymers is advisable to reduce any chemical residues in the effluent which is finally discharged into the environment. Moreover, flocculation is a key process required to settle activated sludge treated effluent solids. Given the well-known discrete motion and relatively heavier BTF effluent solid particles, I consider flocculation process by micro-sand/polymers is considered as an unnecessary technical step in clarifying BTF effluent. For the above reasons I will not recommend ballasted sedimentation system to clarify BTF effluent. Conclusions: 1. I concur with the recommendation made by CH2M Beca to select lamella clarifier as an option to remove BTF effluent solids.

11 Page Whilst facultative ponds can store BTF effluent solids for many years, there are high risks associated with the use of ponds, such as high TSS discharge and potential odour emissions. The above issues combined with the very high footprint I will not recommend ponds. 3. Circular clarifiers are much superior to ponds and well understood and widely used technology, hence can be used for removing solids from the BTF effluent. However, the high compactness of the lamella clarifier enables it an attractive option over circular clarifiers. 4. The use of ballasted sedimentation option is unnecessary to remove BTF effluent solids and the potential use of polymers makes this option inferior to both lamella and circular clarifiers. 5. The designing and installation of any clarifier should be considered after deciding on double BTF and the installation and operation of the additional BTF owing to potential differences in solid loads and characteristics. 6. Any estimates of TSS outputs from BTF effluent must be made from actual BTF outlet flows (preferably discharge flows) rather than the raw wastewater influent flows. This is because owing to BTF wetting requirements of 32,400 m 3 /d, much of the time, the BTF effluent outflow will be significantly greater than the raw wastewater influent flow, unless, the current operation does not require BTF effluent recirculation.