Removal of dissolved reactive phosphorus from municipal and dairy factory wastewater using allophanic soil : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand
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2019
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Massey University
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Abstract
Many of New Zealand’s sewage treatment plants (STPs) and rural factories
discharge treated or partially treated sewage, which is rich in dissolved reactive
phosphorus (DRP), into rivers and streams. A large number of these STPs are
not able to comply with the current DRP river standards because conventional
treatment methods are cost-prohibitive. There is an abundance of Allophanic
soils with high phosphorus (P) sorption capacities located in the central North
Island of New Zealand that have potential for use as low-cost filter material
for removing DRP from wastewaters. For Allophanic soil filters to be a viable
treatment option, the soil, in addition to having a high P sorption capacity,
should be both accessible and plentiful. The main aims of this study were to
assess and improve the effectiveness of Allophanic soil filters at removing DRP
from wastewaters and to evaluate the agronomic value of P-enriched soils as a
P source for plant growth. It also sought to contribute to a better
understanding of the feasibility and important design characteristics of fullscale
soil-based treatment systems.
Five quarry sites in the Waikato Region were soil sampled to identify soils with
high P retention values. Only the Te Mata Quarry (TQ) soil in the, northwestern
Waikato Region, had a high P retention value at or close to 100% as
assessed using the standard (5 g) anion storage capacity (ASC) test. The
modified (1 g) ASC test revealed P retention values of 47 – 91% for samples
taken from different soil depths at TQ. All of the soil depths down to 600 cm,
except for the 125 – 175 cm depth, had modified (1 g) ASC test values >58%.
This indicated that the TQ soil had P sorption capacities that would
potentially make it a suitable material for filtering DRP from wastewater and,
therefore, it warranted further evaluation using real wastewater.
Wastewater pH has a marked influence on the P sorption capacity of soil filters,
with the sorption capacity expected to increase as wastewater pH is decreased,
from being alkaline to acidic. The laboratory soil column experiment quantified
the effect of the level of acid dosing and the type of acid used on the capacity
of soils to remove P from wastewater. Columns of soil, taken from a quarry at
Ohakune (OQ), and treated with wastewater adjusted to pH 5.5 removed the
greatest amount of DRP. A total of 8.9 mg P/g oven-dried soil was removed
at an average removal efficiency of 75%. In comparison, the soil columns
treated with wastewater without pH adjustment, removed only 4.5 mg P/g
oven-dried soil at the same removal efficiency of 75%. This highlights the
merits of lowering wastewater pH to increase DRP removal capacity.
The performance pilot-scale soil filters at the Dannevirke STP and Fonterra
Te Rapa WTP were evaluated, under field conditions, for a total operational
period of 440 and 376 days, respectively. Each filter contained the OQ soil and
had a surface area of 1 m². The OQ soil had an overall P removal efficiency of
67% and 71% at the STP and WTP sites, respectively. The OQ soil filters at
Dannevirke STP removed a total of 6.4 mg P/g oven-dried soil, while the OQ
soil filters at the Fonterra Te Rapa WTP removed a total of 1.87 mg P/g ovendried
soil. This discrepancy in performance was due to the difference in
wastewater type and pH adjustment, initial P concentrations, and soil pretreatment
(i.e. the soil used at Dannevirke was sieved).
A cost/benefit analysis suggested that if the STP was 225 km from the soil
source then the cost of acid dosing is about ten times greater than the cost of
supplying additional soil to achieve the same amount of P removal. Therefore,
it is unlikely that acid dosing will be cost competitive for most wastewater
treatment sites in the central North Island of New Zealand.
The wastewater treated soil (WTS) obtained from the Dannevirke STP pilotscale
filter experiment was evaluated for its agronomic effectiveness in a
glasshouse pot experiment. The ability of WTS to supply P for ryegrass growth
(Lolium multiflorum) was compared with a soluble phosphorus source
(monocalcium phosphate, MCP). The WTS was highly effective at increasing
available P in the soil, as measured by the Olsen P soil test, ryegrass yield and
ryegrass P uptake. The soluble fertiliser P value of WTS was estimated to be
equivalent to 61% of MCP applied at the same rate. Therefore, the results
show that WTS is an effective P source for plant growth and its application to
soil has the potential to recycle both the soil and the P it contains.
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Keywords
Sewage, Purification, Phosphate removal, New Zealand, North Island, Land treatment of wastewater, Soil chemistry