Phosphorus removal in filter plant
Summary and Conclusions
By means of practical tests the project checked two new methods for removal of phosphorus in household wastewater from households situated in a rural zone outside the public sewage catchment area.
The project was realised in the period from September 2000 to September 2001 at three farms.
Purpose
The primary objective was to test whether a phosphorus content of less than 1.5 mg Total-P/l in the treated wastewater can be secured by the two treatment methods.
A sub-objective was to gain experience with simple precipitation equipment to be mounted in settling tanks, including experience with sludge formation and discharge frequency, precipitating phosphorus in these tanks.
Besides, a sub-objective was to gain experience with the establishment, running-in and operation of a Natur-Ren® plant where phosphorus is precipitated in a submerged filter filled with the alternative filtering material Filtraliteâ consisting of crushed burnt clay, dolomite lime and phosphorus-connecting metal ions.
Apart from testing the phosphorus treatment capacity of the filter experiences were gained with the effect of the treatment method on the pH value in the treated wastewater.
Finally, it was a sub-objective to elucidate the capital and operational costs involved in the two treatment methods.
Testing of Simple Precipitation Equipment
Testing of the simple equipment for precipitation of phosphorus was carried out in the course of twelve months in a new-established settling tank equipped with three chambers and in an older settling tank with 2 chambers. By means of pumping and gravitation the wastewater outlets from both settling tanks were subsequently led to a sand filter.
The testing of the precipitation equipment in the new-established settling tank was carried out with only minor and unessential operational irregularities.
Testing the older type of settling tank large quantities of bulking sludge was registered throughout the duration of the test with the result that it was impossible to obtain a reliable basis for assessment of the actual effect of the phosphorus precipitation.
Outlet Quality Precipitating Phosphorus in a New-Established Settling Tank
By adding a precipitant in the new-established settling tank an average outlet quality of 0.4 mg Total-P/l was measured in the outlet from the sand filter in the test period. None of the registered values exceeded 1.5 mg Total-P/l.
Besides, an average outlet concentration of 7.3 mg SS/l was registered indicating good retention of suspended solids through the plant.
The outlet quality of aluminium was 0.16 mg/l on average and, thus, below the limit value of 0.2 mg/l for aluminium in drinking water. Consequently, no appreciably increased washing out of the aluminous precipitant added in the inlet to the settling tank was demonstrated.
In the test period an average outlet concentration of 30.8 mg/l for Total-N was demonstrated. At the end of the test period analyses for nitrification were carried out and complete nitrification was demonstrated in the sand filter, which at that time was approx. 1 year old.
In the test period the pilot plant was loaded with an average of 0.12 m3 of household wastewater per day corresponding to the water consumption of one person. Consequently, the hydraulic wastewater load was approx. 1/3 of the normal load for a household.
During the test and when emptying the sludge volume of the settling tanks no problems with sludge retention were registered. Besides, the volume of floating sludge and bottom sludge was not increased.
Outlet Quality Precipitating Phosphorus in the Older Settling Tank
By adding a precipitant to the older settling tank an outlet quality in the range from 2.3 to 30 mg Total-P/l - with an average of 8.2 mg Total-P/l - was measured in its outlet.
Thus, it was not possible to reduce the phosphorus content to 1.5 mg Total-P/l or below this value. However, in certain periods Ortho-P concentrations of less than 1.5 mg/l were measured indicating that in these periods the phosphorus was precipitated efficiently.
In the outlet from the settling tank an average outlet concentration of 179 mg SS/l was registered indicating washing out of suspended solids (bulking sludge).
This is supported by the fact that the outlet quality for aluminium was measured in the range from 2.4 to 7 mg/l and, thus, above the limit value of 0.2 mg/l for aluminium in drinking water.
Based on the above results it was concluded that the design of the older settling tank was inappropriate for efficient sludge retention and phosphorus precipitation.
Due to the large water depth in the first chamber of the tank optimum mixing of the added precipitant was not achieved. Furthermore, due to the too small volume of the second chamber of the tank sufficient retention of suspended solids could not be secured resulting in bulking sludge and washing out of the precipitant.
In the test period the pilot plant was loaded with an average of 0.4 m3 of household wastewater per day corresponding to the water consumption of three persons. Consequently, the hydraulic wastewater load corresponds to the normal load for a household.
Emptying the settling tanks, an increased volume of floating sludge and bottom sludge could not be observed.
Testing of a Natur-Ren® Plant
Phosphorus treatment in a Natur-Ren® plant was tested in a new-established plant designed according to the manufacturer’s instructions.
The fundamental design of the plant corresponds to a biological sand filter plant, though with the difference that the main part of the filter is submerged (filled with water) and filled with Filtraliteâ (crushed burnt clay with a large content of metal ions and incorporated dolomite lime) whereby the phosphorus content of the wastewater is either bonded or precipitated in this part of the filter.
Outlet Quality Using a Natur-Ren® Plant
The outlet qualities for Total-P and Ortho-P showed that by using the Natur-Ren® plant it was possible to reduce the phosphorus content in the wastewater outlet to an average of 0.1 mg Total-P/l. None of the registered values exceeded 1.5 mg Total-P/l.
In the test period the plant was loaded with an average of 0.29 m3 of household wastewater per day corresponding to the water consumption of 21/4 persons.
During the first part of the test period the outlet quality for suspended solids was placed in the range of 19-500 mg/l characterized by washing out of varying quantities of fine-grained filtering material (filler) and dolomite lime. During the last half of the test period more stable concentrations were measured in the range of 3-28 mg/l demonstrating that the washing out was stagnated.
In the test period an average outlet concentration of 42.6 mg/l for Total-N was demonstrated. The reason for the high value was that in the middle of the test period the outlet of the filter blocked resulting in a rise of the water level in the aerobic part of the filter. This damaged the incipient nitrification in the filter. However, at the end of the test period the test results showed signs of regeneration of an effective nitrification.
The Natur-Ren® plant drastically increased the alkalinity of the treated wastewater and the pH, which rose from approx. 7 to approx. 13. The increase was attributable to the dolomite lime content of the filtering material.
Capital and Operational Costs
The capital costs of a new biological sand filter, including a pumping system and phosphorus precipitation equipment corresponding to treatment class SOP (abbreviation for "Tightened requirement for reduction of organic matter and nitrification, including reduction of phosphorus" according to guidance No. 4 on "Type Approval Scheme for Small Wastewater Treatment Plants" issued by the Danish Environmental Protection Agency in 1999), amount to approx. DKK 45,000, incl. VAT (25%) - though exclusive of the costs relating to the settling tank.
The costs of the phosphorus precipitation equipment amount to approx. DKK 13,000 (construction costs), incl. VAT.
Using the phosphorus precipitation equipment, the current operational costs include electricity consumption and consumption of precipitants; i.e. the yearly costs amount to approx. DKK 150 for electricity and to DKK 350 for precipitants - both incl. VAT.
The total capital costs for a complete Natur-Ren® plant corresponding to treatment class SOP (see abbreviation above) amount to approx. DKK 61,000, incl. VAT but exclusive of the settling tank.
The establishment of a Natur-Ren® plant will not involve operational costs.
The service lives of the two types of plants are estimated at 20 years before the filtering material will have to be replaced.
Provided that the capital investments relating to the plants are depreciated in the course of their service lives and that the operational and maintenance costs are currently included, the yearly average costs for both treatment methods will amount to approx. DKK 5,500, incl. VAT.
Conclusion and Recommendations
Testing in practice the simple precipitation equipment in a new settling tank mounted in front of a new-established biological sand filter plant, an average outlet quality of 0.4 mg Total-P/l was measured in the outlet from the sand filter. None of the registered values exceeded 1.5 mg Total-P/l.
Testing the new-established Natur-Ren® plant, it was possible to reduce the phosphorus content in the wastewater outlet from the plant to an average of 0.1 mg Total-P/l. None of the registered values exceeded 1.5 mg Total-P/l.
The test included three filter plants which gives no statistical security for the general efficiency and reliability of the treatment methods under varying hydraulic and substance load conditions.
However, as the treatment results are considerably below the intended level of 1.5 mg Total-P/l, it may rightly be assumed that the treatment methods can lower the phosphorus content below this level in situations when the hydraulic load and the substance load of the wastewater are higher.
Testing the precipitation equipment, the following experiences were gained:
 | The design, volume and flow conditions of the settling tank are essential for efficient phosphorus precipitation and retention of suspended solids and should as a minimum meet the requirements of guidance No. 3 on "Biological Sand Filters up to 30 PE " issued by the Danish Environmental Protection Agency. |
| The mixing of the precipitant in the clearing zone of the settling tank is decisive for the efficiency of the precipitation. |
| The simple precipitation equipment has performed a reliable dosing and mixing of the precipitant. |
| The phosphorus treatment is efficient shortly after the start-up of the phosphorus precipitation. |
| Precipitating phosphorus in the settling tank of a biological sand filter plant, buffering was registered as a short-time dosing stop of the precipitant did not result in a distinctive increase of the phosphorus content in the outlet from the plant for several days after the dosing stop. |
Emptying the settling tanks of sludge, a distinctively increased bottom sludge quantity could not be observed. Consequently, the test could not with certainty document whether precipitation of phosphorus causes an increased sludge quantity.
The following experiences with respect to establishment, running-in and 1 year of operation were gained during the practical testing of the Natur-Ren® plant:
| An authorised sewer contractor can establish the plant on the basis of the manufacturer’s instructions. |
| The washing out of fine-grained filtering material (filler) and dolomite lime is considerable during the first twelve months after commissioning. This may result in inconvenient deposits in the downstream sewer system if the establishment of a well with a sand / silt trap at the outlet of the filter is omitted. |
| The phosphorus treatment is efficient shortly after the plant has been put into operation. |
| The wastewater is discharged with a pH value of approx. 13. |
The capital costs related to the two types of plants - excl. the settling tank - were estimated as follows:
| Biological sand filter, including a pumping system and phosphorus precipitation equipment - DKK 45,000, incl. VAT; of this the phosphorus precipitation equipment amounts to DKK 13,000, incl. VAT. |
| Natur-Ren® plant - DKK 61,000, incl. VAT. |
Provided that the capital investments relating to the plants are depreciated in a 20-year period - corresponding to service lives of the filtering materials - and that the operational and maintenance costs are included, the yearly average costs for both treatment methods will amount to approx. DKK 5,500, incl. VAT.