DESLUDGING PONDS IN NEW ZEALAND

Transcription

1 DESLUDGING PONDS IN NEW ZEALAND Bridget O Dempsey, Humphrey Archer Beca Infrastructure Ltd, Christchurch, New Zealand (bridget.odempsey@beca.com; humphrey.archer@beca.com ) ABSTRACT Many of the 200 waste stabilisation pond (WSP) systems in New Zealand have been in operation for 30 to 50 years and now require desludging. WSP manuals contain little guidance on sludge removal and handling. This paper draws on our experience of desludging several pond systems around New Zealand, and explores different methodologies for removing, dewatering and disposing the sludge, including beneficial reuse. Cost comparisons and lessons learned from our experiences are described. INTRODUCTION An essential treatment mechanism in a pond is to settle solids, where they accumulate on the base of the pond and form a sludge layer in which Volatile Suspended Solids (VSS), the organic component of the solids, is decomposed by anaerobic digestion. Over a long period of time, the depth of the largely inorganic sludge layer may accumulate to a level where there is insufficient volume in the overlying algae-rich liquid layer for effective treatment. In particular, the insufficient depth of oxygen rich liquid cannot oxidise volatile acids and hydrogen sulphide released from the sludge layer thus causing nuisance odours. At this point the pond needs to be desludged, which is normally after 20 to 30 years of service. The percentage of VSS is generally lower in pond sludge compared to that in sludge derived from other wastewater treatment processes, for example an activated sludge process. This is primarily due to the length of time that the sludge has been stored in the pond, allowing the breakdown of VSS. Die-off of pathogens and viruses also occurs due to the long retention time. LITERATURE REVIEW Practical information on sludge accumulation in ponds, sludge removal and operation of sludge drying basins, is contained in the following references. Pond Treatment Technology, edited by Shilton (2005), contains a brief overview of sludge removal from ponds (p. 271) and a further page on sludge disposal (p. 273). Wastewater Stabilisation Ponds: Sludge Accumulation, Technical and Financial Study on Desludging and Sludge Disposal. Case Studies in France (Picot et al., 2004) describes the sludge accumulation rates in 19 ponds in France, with the average being 18mm/year. Desludging without emptying the pond had an average cost of 62/m³ sludge (at 10% dry solids (DS)) which was 50% greater than if the pond had been emptied, at 42/m³. The sludge was removed at either 6% DS or between 6% and 10%, depending on the haul distance, and carted in manure tankers. The sludge was not dried to about 45% DS, as is done elsewhere. While the paper Improved Design and Operating Criteria for Sludge Lagoons and Drying Pans (Crosher, 2008) deals with sludge from in-tank treatment plants in Victoria, Australia that has been aerobically or anaerobically digested, the data on loading and drying rates in drying pans (beds) can be applied to drying of pond sludge in-situ, or in drying pans receiving dredged sludge. It was found that above 45% DS, the rate of drying is significantly reduced and it is not efficient to attempt to dry to more than 45% DS. The rate of drying was not influenced by the solids loading rate on the pan up to 400 tonne DS/ha. Also the rate of drying is directly proportional to net evaporation, provided the sludge crust is disturbed weekly. The paper Just When is Sludge Material Truly Digested (Crosher and Anderson, 2010) also deals with sludge from in tank treatment

2 plants in Victoria, Australia, that has been aerobically or anaerobically digested. However, the investigations into odour release and volatile solids (VS) reductions in drying pans have applicability to sludge that is dredged from ponds. It was found that when the sludge had dried to above 10% DS, the rate of VS reduction increased sharply and this was attributed to aerobic digestion occurring when oxygen could permeate the sludge matrix. VS contents of less than 20% were achieved. Studies elsewhere have also shown that aerobic digestion after anaerobic digestion results in lower VS contents. When aerobic digestion occurs during drying, odour emissions from the sludge are substantially reduced, thus avoiding odour issues when applying the dried sludge to land, or when stockpiled. This emphasises the need for weekly windrowing of the sludge as it dries, to allow oxygen to penetrate and to achieve aerobic digestion. OPTIONS FOR SLUDGE REMOVAL AND DEWATERING There are two main approaches to sludge removal. These are emptying the pond and solar/air drying the sludge in-situ, followed by removal using an excavator and trucks, or using a dredge to suck the sludge from the base of the pond with external dewatering. If a dredge is used to remove sludge from the ponds, there are a range of methods that could be used to dewater the sludge, including sludge drying beds, centrifuging and geotextile bags. Sludge Removal Options In general, the least expensive option for desludging is to isolate a pond in early summer to allow the sludge to stabilise by anaerobic digestion then to drain the pond, windrow the sludge, and allow it to be dried, by air and solar energy, over the mid to late summer period. It can then be removed by an excavator for disposal or beneficial reuse (see Figure 1). During solar/air sludge drying, the pond is out of service. However, ponds are able to handle higher BOD loads in summer, at least double their normal design loads (refer to Pond Process Design by D. Mara in Shilton, 2005). Taking a pond out of service can be managed if the remaining pond capacity is sufficient to treat incoming sewage (e.g. if there are primary ponds in parallel, or flow can be bypassed to a secondary pond). Figure 1: Solar/air drying sludge at Blenheim STP In some situations, it can be economically viable to construct a new primary pond in parallel, which can then handle the flow and load while the original pond is drained for insitu desludging. The additional pond can then be utilised for population growth, or to restore overall retention times for effective disinfection. In-situ solar/air sludge drying does not work if groundwater is likely to infiltrate the pond, or if there is excessive rainfall in summer. For this option to work efficiently, the groundwater level needs to be sufficiently below the base of the pond. Some types of clay or membrane liners could be susceptible to damage by machinery and precautions need to be taken. Using a dredge to desludge a pond is commonly used in New Zealand, and can be followed by a range of sludge dewatering methods to be used. Usually a contractor will undertake both the desludging of a pond and the dewatering. There are a variety of floating dredges on the market, most of which require an operator on them at all times. Recently, a remotely operated dredge, marketed under the name of SludgeRat by ITT, has become available in New Zealand. This may offer a cheaper solution, particularly for smaller ponds, since less operator time is required. It also has the advantage of not having a cutter, which reduces the potential for damage to the pond liner. The greatest advantage of using a dredge to desludge a pond is that the pond can remain in service while it is dredged. However, care is needed to prevent the suction cutter device touching the liner. The last portion of the sludge will be left in place if there is a liner, mainly so that the clay liner of the pond is not inadvertently dredged out, or a membrane liner is not damaged. Consequently, not all of the sludge in the pond can normally be removed,

3 which can be a significant volume if the base profile is irregular. Sludge Dewatering Options In the case of suction dredging, if there is sufficient space, sludge drying beds can be constructed near the pond being desludged, with the dredge pumping to these (see Figures 2 and 3). The beds are shallow (typically 0.5m sludge depth), with the decant being returned to the pond. Once the sludge has dried sufficiently, it can be removed by excavator. In the case of centrifuging, the dredge pumps directly to the centrifuge, which dewaters the sludge to approximately 20% dry solids. Polymer dosing is required which significantly increases the costs. In each case, the decant is returned to the wastewater treatment plant. Figure 4: Geotube bags after filling (Source: Maccaferri website) OPTIONS FOR SLUDGE DISPOSAL OR BENEFICIAL REUSE AS BIOSOLIDS To reduce transport costs, which are often a significant component of sludge disposal costs, the sludge can be disposed on or near, the wastewater treatment plant site. Options include using the sludge to create a landscaping mound around the wastewater treatment plant, burying the sludge on site, or providing sufficient treatment of the sludge so that it can be reused off-site. Figure 2: Newly constructed sludge drying bed at the Picton STP, showing drain and sump Figure 3: Sludge drying bed at Picton STP (in use) A smaller footprint alternative is for the dredge to pump into geotextile bags, which retain the solids and expel the water, which drains through the geotextile (one proprietary product is Geotube ). Dry solids concentrations of up to 60% can be achieved after one year of storage (see Figure 4). Alternatively, sludge can be disposed to landfill, but its high water content can make it difficult for the landfill operators to work with. This is usually the most expensive option, due to high transport costs and landfill charges. In New Zealand, if sludge is to be considered for beneficial reuse on land, it needs to be classified as biosolids which is a term used to describe sludge that has undergone further processing to improve its quality, normally to a specified standard or class or grade. The quality of the biosolids depends on the wastewater and solids treatment processes utilised. A grading system is given in the Guidelines for the Safe Application of Biosolids to Land in New Zealand (NZWWA, 2003) to identify biosolids quality and the various treatment stages required to obtain a standard biosolids grade. The grading system generally follows the examples in the USA and Australia, but appears to have more stringent limits for some contaminants. The New Zealand biosolids grading system is made up of two parts. The first is the stabilisation grade, denoted by A or B. This

4 is dependent on the organic matter and pathogen content. The stabilisation grade depends on whether or not an approved pathogen reduction procedure and an approved vector attraction reduction (VAR) method have been implemented. Vectors include any animal or insect that is a potential carrier of disease, which can transmit pathogens to humans or other hosts physically through contact or by playing a specific role in the lifecycle of a pathogen. In general, stabilisation Grade A means the biosolids have very few pathogens and is suitable for use by general public and in public areas with no access restrictions, although this is subject to Regional Plan rules which vary across New Zealand. Stabilisation Grade B means the biosolids are not suitable for use by the general public and requires restrictions on access to sites or incorporation into soil, with such normally requiring a resource consent. For pond sludge to comply with Grade A, this would require stockpiling of dried sludge for one to three years so that the pathogen counts are reduced sufficiently to comply with the Grade A guidelines. The second part, denoted by a or b, is the chemical contaminant grade, and is determined by the concentrations of metals and Persistent Organic Pollutants (POPs) such as cadmium, copper, mercury, chlordane and dioxin in the biosolids. The biosolids are graded a if all metal and POP concentrations are below the prescribed limits. If one contaminant concentration exceeds the prescribed limit the biosolids are then graded as b, unless any one contaminant concentration exceeds the prescribed limits for a b grading, in which case the biosolids will be classed as sludge, which is not suitable for application to land. Sludge quality data for a New Zealand pond in a mainly domestic catchment is shown in Table 1, with values shown in bold where the Grade a biosolids limit is exceeded. These test results show that the pond sludge would be Grade b biosolids, but because of near compliance, could become Grade a, if bulked with bark, compost or sand. The critical contaminants are those typically found in domestic catchments, with copper and zinc from piping systems and roofs, and mercury from dental fillings and dental premises. Arsenic is a commonly found contaminant in New Zealand waters deriving from volcanic soils. CASE STUDIES Before being upgraded in 2008, the Patea Sewage Treatment Plant (STP) in Taranaki consisted of a single oxidation pond. A sludge survey showed that sludge occupied 31% of the total pond volume, with an average depth of 0.32m, compared with an average pond depth of 1.03m. Clear water depths of less than 0.75m were recorded in the middle of the pond (MWH, 2004). As part of the upgrade to three ponds in series, half of the pond was desludged. The sludge was deliberately left in the other half, to avoid an increase in exfiltration through the base of the pond, which could destabilise the cliff top on which the pond sits. Solar drying was attempted over the first summer, but this was unsuccessful due to wet weather. The pond was dredged and dewatered using a centrifuge. Protests by the local community prevented disposal of the sludge at the town s landfill, so it has been stored in a lined and covered corner of the pond until a district-wide sludge disposal site can be secured. 263 tonnes (1,315m³) of sludge were removed at a cost of $537 per tonne of dry solids, or $107/m³. The approximate area desludged was 1.24ha, so this equates to $113,000 per ha of pond desludged. The Manaia STP (Taranaki) pond was dredged, dewatered and incorporated into the base of new wetlands to prevent exfiltration. 311 tonnes (1,555m³) of sludge were removed at a cost of $977 per tonne of dry solids, or $195/m³. The pond area was 1ha, so this equates to $304,000 per ha of pond desludged. The higher cost when compared with Patea STP was due to a stand-down period while the disposal site was changed. The Eltham STP (Taranaki) pond was dredged and sludge dewatered by geotextile bags, which were then buried on site (see Figures 5 and 6). 2,400m³ of sludge at 18% dry solids was dredged and dewatered in the geotextile bags, at a cost of $244,000, which equates to $565 per tonne of dry solids, or $102/m³. The area of the pond which was desludged is 3ha, so this equates to a cost of $81,000 per ha. The Blenheim STP (Marlborough) pond was drained and air/solar dried over summer (see Figure 1). The original pond could be drained because two new primary ponds had been constructed over the preceding summer, to provide additional capacity. A dry solids content of approximately 50% was achieved after some windrowing. The dried sludge was

5 stored in a sludge landfill formed at one end of the pond. The cost was $250,000 for a 16ha pond in 2000, or $15,625 per hectare. In 2011 dollar values, this would be $20,500 per hectare. Geotubes WSP Figure 5: Eltham STP waste stabilisation pond and getotextile bags Figure 6: Geotextile bags at the Eltham STP, before filling For the Masterton STP (Wairarapa), new ponds are currently being constructed on higher ground further from a river to avoid potential river erosion and earthquake damage. The three existing ponds which have been in service for 40 years, will be drained and desludged by in-situ air/solar drying with the dried sludge being moved by excavator and trucks and placed in an on-site sludge landfill. The existing pond area will be converted to pasture and irrigated with pond effluent. The tender price for the desludging operation is $11/m³ (dried volume of sludge in landfill). The total price, including construction of the silty clay lined and capped landfill, is $22/m³. This equates to $30,800 per hectare of pond desludged, which compares well with the Blenheim example above, when different standards of sludge landfilling and risk of flooding by the adjoining river are considered. For the Waimate STP (South Canterbury), the original single pond was drained after new maturation ponds had been constructed and raw sewage was temporarily diverted to the maturation ponds. Sludge was dried over summer and stored in a borrow pit on site which had been used to supply clay for the new maturation pond construction. The Christchurch Wastewater Treatment Plant (WWTP) (Canterbury) has a 225ha maturation pond system after an in-tank treatment train based around trickling filters. Pond 1 (33ha) and Pond 2 (58ha) were desludged over two summers by air/solar drying in-situ. Dried sludge was stored in landfill areas formed at the western areas of each pond. The sludge landfills were intended to strengthen the original perimeter banks which were constructed from sand about 1960 and were assessed as likely to fail in a significant seismic event, causing flood damage to nearby houses and a major electrical substation (220 kv). The major Magnitude 7.1 earthquake which occurred on 4 September 2010 and subsequent Magnitude 6.3 earthquakes on 22 February 2011 and 13 June 2011, did not cause significant damage to the sludge landfill and associated banks. In comparison, other pond banks had substantial damage (refer to the paper Christchurch Maturation Ponds Upgrading and Earthquake Impacts by O Dempsey, Feary & Archer also being presented at this conference). The Mangere WWTP (Auckland) ponds at 530ha, were the second largest in the world after the Werribee (Melbourne) ponds which cover 2,500ha. The Mangere ponds were decommissioned in 2002/03 and the area has reverted to tidal flats as part of Manukau Harbour. To avoid negative environmental effects on the harbour waters, the ponds were dredged and the sludge dewatered by centrifuges and polymer dosing. Dewatered sludge was stored in a portion of one of the ponds. Sludge from mechanical wastewater treatment plants often create nuisance and high costs for treatment and disposal. However, at the Picton

6 STP, waste activated sludge is fed to two 5000m3 sludge lagoons for stabilisation by anaerobic digestion at ambient temperatures (see Figure 7). The surface layer in the lagoons is rich in algae that produce oxygen. The oxygenated surface layer treats potentially odorous gases emitted from the digestion processes in the lower zone. A 2.2kW brush aerator is installed in each lagoon to provide extra oxygen when required and to avoid stratification. The lagoons are operated to an annual cycle, alternately filled with waste solids each year, then rested for a few months before being drained. Stabilised solids are either pumped (at about 5% dry solids) or removed by excavator (at about 10% dry solids) to two drying basins formed in the adjoining borrow pit (see Figures 2 and 3). After drying by solar and air processes to at least 50% dry solids, the biosolids are then transferred to stockpiles on the adjoining closed landfill. Storage for at least one year further dries the product and reduces pathogen content so that the biosolids are available for use in Council landscaping projects. Odour has not been an issue for any of the above case studies. groundwater levels, and also occupying a smaller footprint. ACKNOWLEDGEMENTS Thank you to Vikki Kuyl from South Taranaki District Council, for providing background information. REFERENCES CH2M Beca Ltd (2011). Desludging and Disposal Options for Kaiapoi and Rangiora Wastewater Treatment Plant Ponds, report prepared for Waimakariri District Council. Crosher, S. (2008). Improved Design and Operating Criteria for Sludge Lagoons and Drying Pans. In Proc 71 st Annual Water Industry Conference, Bendigo, Australia, September Crosher, S. and Anderson, T. (2010). Just When Is Sludge Truly Digested. AWA Biosolids Specialty Conference, Sydney Australia, June MWH (2004). Oxidation Pond Sludge Surveys Eltham, Manaia, Patea, prepared for South Taranaki District Council. O Dempsey, B.M., Feary, J. & Archer, H.E. (2011). Christchurch Maturation Ponds Upgrading and Earthquake Impacts. 9 th IWA Specialist Group Conference on Waste Stabilisation Ponds, Adelaide. Figure 7: Picton STP, with sludge lagoons at rear CONCLUSIONS Desludging a pond by taking it out of service, draining and using solar/air drying to dewater the sludge is the least expensive option, but is only practical where there is sufficient capacity elsewhere in the STP to enable the pond to be bypassed. In addition, environmental factors such as dry weather over summer and groundwater levels below the base of the pond, are also requirements. Picot B et al. (2004). Wastewater Stabilisation Ponds: Sludge accumulation, technical and financial study on desludging and sludge disposal. Case studies in France. 6 th International WSP Conference, Avignon France, IWA, Sept Shilton, A. (ed.) (2005). Pond Treatment Technology, IWA Publishing. More expensive options are desludging using a dredge, and dewatering using geotextile bags or centrifuge. These have the advantage not being affected by inclement weather or

7 Table 1 Pond Sludge Quality Data Parameter Pond Sludge Grade a NZ Grade b NZ Example Biosolids Limit Biosolids Limit Arsenic (mg/l) Cadmium (mg/l) Chromium (mg/l) ,500 Copper (mg/kg dry solids (DS)) ,250 Lead (mg/kg DS) Mercury (mg/kg DS) Nickel (mg/kg DS) Zinc (mg/kg DS) ,500 DDT/DDD/DDE (mg/kg DS) Dieldrin (mg/kg DS) Aldrin (mg/kg DS) < Total PCB (mg/kg DS) < Chlordane (mg/kg DS) < Heptacholor and Heptacholor Epoxide (mg/kg DS) < Hexachlorobenzene (mg/kg DS) < Lindane (mg/kg DS) < Benzene hexachloride (mg/kg DS) <